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 He had not sought in his mind for the flood's cause, but had merely taken it numbly as part of the fury and confusion of a world in ruin. Anyway, he was dead tired out on his feet. the slime that covered a surface faintly, horrifyingly resilient. He recoiled as if seared, and retreated, slithering in the muck. For could see the rest—the immense slug-like shape that extended in ponderous repose across the river valley, its head and tail spilling over the hills on either side, five miles apart. The beast was quiescent until morning—sleeping, if such things slept. And that explained the flood the monster's body had formed an if it did not move until dawn, the level would be far higher then. Westover stood motionless in the blackness how long, he did not know. He was hardly aware of the water that covered his feet, crept over his the monster's foot, and sought above him with upstretched hands—found but he could hear the wash and chuckle of the flood below. His tired brain told him treacherously: "I'm already minutes after he had found a new foothold merely clung panting with Some time after he had found courage to resume the climb, he dragged which he had to make before dawn ... but not now ... not now.... 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 expanded under the noonday sun, creamy white above and black and oily below, and beneath the black cloud something that writhed and flowed sluggishly in a cyclopean death agony. That picture dissolved, and was replaced by the face of a man—one who might now be alive or dead, elsewhere in the chaos of a desolated planet. It was an ordinary face, roundish, spectacled, but etched now by tragedy the voice that went with it was flat, unemotional, pedantic. "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 structure, the monsters have attained their great size and perhaps for "The monsters who have descended on Earth must represent the end result surface systematically ingesting all edible material—all life not 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 monsters have finished with Earth, there will be no vegetation left all devoured by the monsters." Westover awoke, feeling himself bathed by the cold sweat of nightmare—then he realized that a misty rain had wetted his face and He sat up, eyes searching the sky for a sign to tell him how long he monster would begin to move, to crawl eastward impelled by the same dim around the Earth, gutting the continents and churning the sea bottoms, and by now whatever was left of human civilization must be starving beyond the Arctic circle, or aboard ships at sea. The hordes that this—would not live long. 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 He could stay where he was unharmed, of course. On the monster's back, of all places, he had nothing to fear from it or from others of its kind. But he knew with desperate clarity that by nightfall, when the beast became still once more, exhaustion and growing hunger would have Again he lay half conscious, in a lethargy that unchecked must grow steadily deeper until death. Isolated thoughts floated through his head. It occurred to him that he was now ideally located to conduct the experiments necessary to prove his theory of how to destroy the laboratory on the monster's back. Of course the rolling motion would to glimpse Sutton's face, as the biologist calmly made that grisly report to the President's Committee on Extermination.... Sutton's prediction had been a hundred percent correct. The monsters' hunger knew no halt until they had absorbed into themselves all the organic he was starving now.... A ripple that did not belong to the crawling motion ran over the like a flea, he had lodged on a larger animal and was about to nourish himself from it. The slabs of flesh he had cut off were gray and unappetizing, but he knew from the studies he had helped he made a smoldering fire from the loose fibrous scale of the monster's back, and half an hour later was replete. Either the long fast, or involuntary revulsion, or perhaps merely the motion of the creature brought on nausea, but he fought it sternly back and succeeded in keeping his strange meal down. Then he was tormented by thirst. It was some time, though, before he on a dog. The monster crawled by day and rested by night not. It wasn't, he argued with himself sometimes in the days when he lay torpidly drowsing, lulled by the long sway, arms over his head to was conducting an experiment.... His life on the monster's back was proving something, something of vast importance for man, the extinct animal—but for increasingly longer periods of time he could not remember what it was.... There came a morning, though, when he remembered. Thus began for him a weird existence—the life of a parasite, of a flea on a dog. The sun was already up, and the monster should have begun once more its steady, ravenous march to the east. But there was no motion the great living expanse lay still around him. He wondered wildly if it was dead. it was as though the cessation of the rock and sway had exorcised the lethargy that had lain 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 because of their gaudy cellulose-plastic armor.... The monster on which Westover had been living as a parasite was generating gases within itself, preparing to leave the ravished Earth. That was the meaning of its gargantuan belly rumblings. And they meant further that he must finally leave it—now or never—or be borne aloft to die gasping in the stratosphere. of the sea. While he slept the monster had crept beyond the coast Of course—the great beast would crawl into the sea, which would float beasts fought crazily over the last morsels of food. He had lost track of the days he had been on the monster's back, but the rape of Earth Then he sprang to his feet, gripping his hand-ax, and ran panting to the place where he had dug for food. His excavations tended to close and heal overnight now he went to work with vicious strokes enlarging When, a few hundred yards nearer the monster's head, the other halted The crawling descent through the twisting, Stygian burrow had much "I stumbled onto it," Westover admitted. "I was wandering across country—my plane crashed on the way back from that South American bug hunt dreamed up by somebody who'd been reading Wells' War of the had a sure-fire way to do that—but I didn't realize then that it was too late to think of killing them off." Sutton nodded thoughtfully. "It was too late—or too early, perhaps. from the beginning. But the monsters absorbed into themselves all the 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 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 did the monster stop crawling by day?

<options>:
A It was no longer hungry
B It was ready to leave Earth
C The sun was up
D It was dead

<answer>:

B