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

<passage>:
for weapons more adequate for encountering the local fauna when it was over. Blast-rifles were not effective against such creatures as these. Torches were contact weapons It seemed insane that such creatures, even in miniature, should painstakingly be brought across light-years of space to the new worlds men settled on. But it had been found to be necessary. The ecological system in which human beings belonged had turned out to be infinitely complicated. It had turned out, in fact, to be the ecological system of Earth, and unless all parts of the complex were present, the total was subtly or glaringly wrong. So mankind distastefully ferried pests as well as useful creatures to its new worlds as they were made ready for settlement. Mosquitos throve on the inhabited globes of the Rim Stars. Roaches twitched nervous antennae on the settled planets of the Coal-sack. Dogs on Antares had fleas, and scratched their bites, and humanity spread through the galaxy with an attendant train of insects and annoyances. If they left their pests behind, the total system of checks and balances which make life practical would get lopsided. It would not maintain itself. The vagaries that could result were admirably Nadine . Something had been left out of the seeding of this planet. The element—which might be a bacterium or a virus or almost anything at all—the element that kept creatures at the size called "normal" was either missing or inoperable here. The results were not desirable. much. A water-ice ice-cap said that there were no poisonous gases in the planet's atmosphere. Sulfur dioxide or chlorine, for example, would not allow the formation of water-ice. It would have to be sulphuric-acid or thick, dense cloud-masses which covered most of the surface,—all the , then approaching the world on planetary drive. He was to be left here, with no reason ever to expect rescue. Two of the four-man crew watched out the same ports as the planet seemed to approach. Burleigh said encouragingly such as are placed on certain worlds for the convenience of interstellar skippers who need to check their courses on extremely long runs. This was something else. its space-port. With clearance-papers in order, they could land unquestioned at any other space-port and take off again—provided the of space-travel, any criminal anywhere could escape the consequences of have bought a ticket, but he'd tried to get off the planet Coryus on the Coryus Three. The others were fugitives too, from some unnamed world Moran did not know. They might be sent back where they came from. In effect, with six people on board instead of five, the practically any direction for a length of time that was at least indefinite. A liner could re-locate itself without trouble. It had elaborate observational equipment and tri-di star-charts. But smaller craft had to depend on the Galactic Directory. The process would be to find a planet and check its climate and relationship to other planets, and its flora and fauna against descriptions in the Directory. That was enough for ourselves. But we can spare some. We'll give you the emergency-kit, anyhow." The emergency-kit contained antiseptics, seeds, and a weapon or two, with elaborate advice to castaways. If somebody were wrecked on an even possibly habitable planet, the especially developed seed-strains would provide food in a minimum of time. It was not an encouraging thought, very important, and the fact that Moran had forced him to fight and killed him in fair combat made no difference. Moran had needed to get off-planet, and fast. But space-travel regulations are especially designed to prevent such escapes. He'd made a pretty good try, at that. One of the controls on space-traffic required a ship on landing to deposit its fuel-block in the space-port's vaults. The fuel-block was not returned until clearance for departure had been granted. But Moran had waylaid the messenger carrying the planet from which they'd carried away a criminal, even though they'd done it unwillingly. Investigation of such a matter might last for months. The Nadine checked her downward motion. Interplanetary drive is rugged and sure, but it does not respond to fine adjustment. Burleigh used shared the peculiarity of the ground as far as they could see through the haze. It was not vegetation in any ordinary sense. Certainly it was no mineral surface! The landing-pockets had burned away three or four spare. And meanwhile we'll take a look at that wreck yonder. There might be an indication in it of what solar system this is. There could be something in it of use to you, too. You'd better come along when we suit. Modern space-suits weren't like the ancient crudities with bulging metal casings and enormous globular helmets. Non-stretch fabrics took the place of metal, and constant-volume joints were really practical suits would take care of that. Anyhow the ice-cap said there were no water-soluble gases in the atmosphere, and a gas can't be an active poison if it can't dissolve. They filed out of the airlock. They stood on ash-covered stone, only slightly eroded by the processes which made life possible on this The hole exposed a cheesy mass of soft matter which seemed riddled with small holes. " They're—bugs! " she said incredulously. " They're beetles! They're twenty times the size of the beetles we humans have been carrying around the galaxy, but that's what they are! " discovered. It had not been practical for men to move onto new planets and subsist upon the flora and fauna they found there. On some new planets life had never gotten started. On such worlds a highly complex operation was necessary before humanity could move in. A complete ecological complex had to be built up microbes to break down the rock for soil, bacteria to fix nitrogen to make the soil fertile plants to grow in the new-made dirt and insects to fertilize the plants so they would multiply, and animals and birds to carry the seeds planet-wide. On most planets, to be sure, there were local, aboriginal plants and animals. But still terrestrial creatures had to be introduced if a colony was to feed itself. Alien plants did not supply satisfactory food. So an elaborate adaptation job had to be done on every planet before native and terrestrial living things settled down together. It wasn't impossible that the scuttling things were truly beetles, grown large and monstrous under the conditions of a new planet. And the ground.... "This ground stuff," said Moran distastefully, "is yeast or some sort of it, so somebody dumped germs and spores and bugs to make it ready for plants and animals eventually. But nobody's come back to finish up the job." "It's not a hunting creature on worlds It was to be noted that Hallet the engineer, avoided the huge harmless creature more widely than most. everywhere. "We're going to find that this wreck has been here a century creatures in their labyrinths of tunnels began to panic. Off to the right the blanket-like surface ripped and they poured out. They scuttled crazily here and there. Some took to wing. By instinct the other men—the armed ones—moved back from the smoke. They wore space-helmets But above all he raged because he was to be marooned here. He could not altogether blame the others. They couldn't land at any colonized world with him on board without his being detected as an extra member of the crew. His fate would then be sealed. But they also would be investigated. Official queries would go across this whole sector of the galaxy, naming five persons of such-and-such description and A ship that came in to port with two few on board would be investigated as thoroughly as one that had too many. Perhaps more thoroughly. So if

<question>:
Why have humans been carrying beetles around the galaxy?

<options>:
A Beetles are just one piece of the puzzle to create an ecological system capable of supporting human life.
B Beetles are a good source of protein for new planet settlers.
C Beetles, like other pests, always seem to make their way on board spaceships.
D Beetles are an integral part of a spaceship's waste management system.

<answer>:

A