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

<passage>:
PLANET of DREAD By MURRAY LEINSTER Illustrator ADKINS The thing presumably died, but it continued to writhe senselessly. He turned to see other horrors crawling toward him. Then he knew he was being marooned on a planet of endless terrors. 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 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 system had been a vast glaring disk off to port, with streamers and prominences erupting about its edges. Now it lay astern, and Moran could see the planet that had been chosen for his marooning. It was a cloudy world. There were some dim markings near one lighted limb, but nowhere else. There was an ice-cap in view. The rest was—clouds. The ice-cap, by its existence and circular shape, proved that the planet rotated at a not unreasonable rate. The fact that it was water-ice told . Something had been left out of the seeding of this planet. The element—which might be 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 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 told about temperature-distribution on the planet. A large cap would have meant a large area with arctic and sub-arctic temperatures, with hydrochloric-acid ice. But the ice-cap was simple snow. Its size, too, allow the formation of water-ice. It would have to be sulphuric-acid or small temperate and tropical climate-belts. A small one like this meant 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 speaker. It wasn't a voice-signal. It wasn't an identification beacon, 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. indefinite distance in an indefinite direction from their last unquestioned at any other space-port and take off again—provided the other space-port was one they had clearance for. Without rigid control of space-travel, any criminal anywhere could escape the consequences of 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 the way to find out where one was, when one's position became doubtful. The Nadine needed to make a planet-fall for this. innumerable random noises to be heard in supposedly empty space. "That's a marker," Carol announced. "I saw a costume-story tape once that had that sound in it. It marked a first-landing spot on some planet or other, so the people could find that spot again. It was supposed to be a long time ago, though." signal. Please reply." "I think," said Carol, "that we should land. People have been here. If they left a beacon, they may have left an identification of the planet. had cleared for Loris. That was where it should make its next landing. The little yacht went on. All five of its proper company watched as the planet's surface enlarged. The ice-cap went out of sight around the bulge of the globe, but no markings 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, though, and Moran grimaced. 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 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. surface that the planet-wide cloud-level seemed to begin. From there on down it was pure opacity. Anything could exist in that dense, almost distance, and not at all beyond a quarter-mile or so. vegetation on a planet with a sol-type sun. Harper spoke from the direction-finder "The signal's coming from that mound, yonder." the only considerable landmark within the half-mile circle in which The Nadine checked her downward motion. Interplanetary drive is rugged very much like a small boy trailing a stick against a picket fence, only ash-covered stone on which the rested. The enigmatic, dirty-yellow-dirty-red-dirty-blue-and-dirty-black ground-cover hid spare. And meanwhile we'll take a look at that wreck yonder. There might displayed in every action. planet. They looked dubiously at the scorched, indefinite substance which had been ground before the Nadine stone on which the obvious panic. They popped out everywhere. It was suddenly apparent that the top of the soil, here, was a thick and blanket-like sheet over the whitish stuff. The black creatures lived and thrived in tunnels under it. 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 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 by reaching forward with its fore-part, securing a foothold, and then harshly to the men with him "It's not a hunting creature on worlds where it's smaller. It's not likely to have turned deadly here. Come on!" 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 such-and-such fingerprints, voyaging in a space-yacht of such-and-such size and registration. The world they came from would claim them as " Look out! It's coming! Kill it! Kill it—. " , necessarily accepted as a member of her crew.

<question>:
What, within the context of the passage, is a 'marker'?

<options>:
A A microchip inserted into a person, designating them as a fugitive
B A safe space for a spacecraft to land on an alien planet
C A sound picked up on a radar that reveals the closest habitable planet
D A location on the alien planet that indicates high predatory activity

<answer>:

B