Read the following passage and questions, then choose the right answer from options, the answer should be one of A, B, C, D.

<passage>:
Gigantic, mindless, the Monsters had come out of interstellar space to devour Earth. They gnawed And that explained the flood the monster's body had formed an the monster's foot, and sought above him with upstretched hands—found steep that rose to the summit of the monster's humped back, a mountain He lay in a state between waking and dreaming, high on the monster's side and it seemed that the colossal body moved, swelling and sighing—but he knew they did not breathe as backboned animals do. Westover had been one of the men who, in the days when humanity was still fighting, had accumulated quite a store of knowledge about the enemy—the enemy that was brainless and toolless, but that was simply too vast for human intelligence and weapons to defeat.... "There are so many of them, and we've destroyed so few—and to kill those few took our mightiest weapons. Examination of the ones that have been killed discloses the reason why ordinary projectiles and bombs and poisons are ineffective against them—apart, that is, from the chief reason of sheer size. The creatures are so loosely organized that a local injury hardly affects the whole. In a sense, each one of them is a single cell—like the slime molds, the Earthly life forms that most resemble them. "That striking resemblance, together with the fact that they chose Earth to attack out of all the planets of the Solar System, shows they must have originated on a world much like this. But while on Earth the slime molds are the highest reticular organisms, and the dominant life is all multicellular, on the monsters' home world conditions must have favored unicellular growth. Probably as a result of this unspecialized structure, the monsters have attained their great size and perhaps for the same reason they have achieved what even intelligent cellular life so far hasn't—liberation from existence bound to one world's surface, the conquest of space. They accomplished it not by invention but by adaptation, as brainless life once crawled out of the sea to conquer the dry land. "The monsters who have descended on Earth must represent the end result of a long evolution completed in space itself. They are evidently deep-space beings, able to propel themselves from planet to planet and from star to star in search of food, guided by instinct to suns and mobile enough to avoid their march. They are like caterpillars that overrun a planet and strip it of its leaves, before moving on to the next. "Man is a highly mobile species, so our direct casualties of this invasion have been very light and will continue to be. But when the monsters have finished with Earth, there will be no vegetation left all devoured by the monsters." idea for stopping the monsters. monster would begin to move, to crawl eastward impelled by the same dim For a man like Westover, who had been a scientist, it was not the prospect of death that was most crushing, but the death blow to his human pride, the star-storming pride of mind and will—defeated by sheer bulk and mindless hunger. Near the crest of the monster's back, he stumbled and fell hands and knees on the shagreen-roughness of the skin beast became still once more, exhaustion and growing hunger would have the experiments necessary to prove his theory of how to destroy the monsters—if only someone had had the foresight to build a biological laboratory on the monster's back. Of course the rolling motion would prediction had been a hundred percent correct. The monsters' hunger knew no halt until they had absorbed into themselves all the organic material on the world which was their prey.... And men must starve, as hack with feverish industry at the monster's crusted hide. the monster's flesh. were gray and unappetizing, but he knew from the studies he had helped Sutton make that the monsters, extraterrestrial though they were, were in the basic chemistry of proteins, fats and carbohydrates one with man or the amoeba, and therefore might be—food. fire from the loose fibrous scale of the monster's back, and half an fought it sternly back and succeeded in keeping his strange meal down. on a dog. The monster crawled by day and rested by night was conducting an experiment.... His life on the monster's back was Now he listened strainingly to the portentous sounds of change in the monster's vitals, and in a flash of insight knew them for what they were. The scientists had found, in the burst bodies of the Titans that had been killed by atomic bombs, the answer to the riddle of that in the living animal could be under exceedingly high pressures, and that could be expelled to drive the monster in flight like a reaction engine. Rocket propulsion, of course, was nothing new to zoology it was developed ages before man, by the squids and by those odd degenerate relatives of the vertebrates that are called tunicates because of their gaudy cellulose-plastic armor.... That was the meaning of its gargantuan belly rumblings. And they meant its bloated bulk and enable it to accelerate and take flight. It would never have been able to lift itself into the air from the dry land. of the days he had been on the monster's back, but the rape of Earth one-willed swarm that Earth's astronomers had at first taken for a faint eructations deep inside the monster. and heal overnight now he went to work with vicious strokes enlarging the latest one, hacking and tearing it deeper and deeper. When, a few hundred yards nearer the monster's head, the other halted demonologist's imagination could have conceived without experiencing rank with the familiar heavy sweetish odor of the monster's colorless try out my method of killing the monsters. I thought—I still think—I from the beginning. But the monsters absorbed into themselves all the source—the monsters themselves. "The monsters almost defeated us, because of their two special adaptations of extreme size and ability to cross space. But man has always won the battle of adaptations before, because he could improvise new ones as the need arose. The greatest crisis humanity ever faced running by tapping the monster's internal gas pressure. There are problems left before we'll be fully self-sufficient here—but the monster is so much like us in fundamental makeup that its body contains all the elements human life needs too." children's—as parasites inside this monster and whatever others we can you mentioned—your monster-killing scheme." from an endoparasite's point of vantage, merely by isolating from the creature's blood over a long period enough of some potent secretion—hormone, enzyme or the like—to kill when suddenly reintroduced into the system. "Originally I thought we could accomplish parasites. That means before all, for the coming generations, that we keep our numbers down. A good parasite doesn't destroy or even overtax its host. as the bugs of bubonic plague or typhoid "Your idea is dangerous for the same reason. The monsters probably spend thousands of years in interstellar space during that time they'll be living exclusively on their fat—the fuel they stored on Earth, and so will we. We've got a whole new history of man ahead of us, under such changed conditions that we can't begin to predict proliferate until they kill their hosts. But imagine a struggle for Lebensraum when all the living space there is is a few thousand monsters capable of supporting a very limited number of people each—with your method giving an easy way to destroy these little Eventually, the monsters will come to another planet not too unlike Earth, because it's on such worlds they prey. A tapeworm can cross the His voice was drowned in a vast hissing roar. An irresistible pressure distorted the walls of the chamber and scythed its occupants from their

<question>:
Why are the monsters so difficult to kill?

<options>:
A They're so large that they'll regularly flip over and crush any humans that are riding on them
B They're so large that they're generally undisturbed by injuries
C They're so large and common that humans have to move only by riding on them or jumping from monster to monster
D They're large and they're so evolved that they can regenerate body mass and heal themselves

<answer>:

B