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

<passage>:
stars shone dimly. Getting through that dome had been his first urgent problem, but now he The man was waiting for him at the mouth of the ascending tunnel. Syme looked at him casually, without a flicker of expression, and "Why, no," Syme told him bewilderedly. "I haven't been following you. Syme stooped over him swiftly, found a thick wallet and thrust it into slipped free, and the tearing, unbearable weight vanished. The reaction swung Syme against the building again, and he almost lost his slippery hold on the balustrade. After a moment he heard the He heard running footsteps, and then a pale face peered over the ledge at him. He realized suddenly that the whole incident could have taken only a few seconds. He croaked, "Get me up." dark, lean face. "Thanks for giving me a hand." "You scared hell out of me," said the man. "I heard a thud. I thought—you'd gone over." He looked at Syme questioningly. found until morning. need a guide. D'you know this part of Mars well?" in its influence, propelling themselves by the motion of their delicately-webbed feet and trailing long gauzy streamers of synthesilk after them. Syme watched them through narrowed lids, feeling the glow of culcha Harold Tate told him, and later, when Syme had taken him to his rooms, the artifacts, the records of the Martians were all there, perfectly preserved inside the city's bubble-like dome, after God knew how many thousands of years. But they couldn't be reached. For Kal-Jmar's dome was not the thing of steelite that protected as it defied explosives and diamond drills. The field extended both above and below the ground, and tunneling was of no avail. No one knew what had happened to the Martians, whether they were the ancestors of the present decadent Martian race, or a different species. No one knew from every angle, taken photographs of its architecture and the robots had tried everything they knew to pierce the wall. Later, however, when every unsuccessful attempt had precipitated a bloody uprising of the present-day Martians—resulting in a rapid dwindling of the number of Martians—the Mars Protectorate had stepped Tate, a physicist, had stumbled on a field that seemed to be identical hours, by the blindest of chances, blurted out his secret to Syme "Why. Are they really dangerous?" "They're unpredictable," Syme told him. "They're built differently, and they think differently. They breathe like us, down in their caverns where there's air, but they also eat sand, and get their oxygen that way." "Yes, I've heard about that," Tate said. "Iron oxide—very interesting deserts—all full of deadly poisons, from arsenic up the line to xopite. They seem intelligent enough—in their own way—but they never come near our cities and they either can't or won't learn Terrestrial. When the first colonists came here, they had to learn might do anything," Syme said curtly. "Don't worry about it." The hills were much closer than they had seemed, because of Mars' sliding treads up the sides of steep hills only to slither down again on the other side. revealed itself to be attached to a length of thick, very strong wire cable, with a control cord inside. They inched down the almost vertical incline, unreeling the cable behind them, and starting minor landslides as they descended. Neither Syme nor Tate ever had a chance to test the efficiency of their harpoon gun. They had traveled no more than five hundred meters, and the gully was as deep as ever, when Tate, looking up, saw a deeper Syme was cursing slowly and steadily with a deep, seething anger. Tate caught a glimpse of the horde of beasts that were rushing up the gully toward them. "My God!" he said. "What are those?" Syme looked. "Those," he said bitterly, "are Martians." The natives, like all Martian fauna, were multi-legged. Also like all Martian fauna, they moved so fast that you couldn't see how many legs as they appeared, being collapsed at the moment. What caused the bulge that made their torsos look like sausages was a huge air bladder, with a valve arrangement from the stomach and feeding directly into the bloodstream. Their faces were vaguely canine, but the foreheads were high, and the helped to take care of the extremes of Martian temperature. Right now they were mostly black. The natives slowed down and spread out to surround the wrecked sand Martians. Syme stopped cursing and watched tensely. Tate said nothing, but he One Martian, who looked exactly like all the rest, stepped forward and "What do you think they'll—" he began, and then stopped himself. "I know. They're unpredictable." "Yeah," said Syme, and opened the door. The air in the car into the near-vacuum outside, and he and Tate stepped out. They bounded along for what Syme figured as a good kilometer and a half, and they then reached a branch in the gully and turned down it, going lower all the time. Under the light of their helmet lamps, they could see the walls of the gully—a tunnel, now—getting darker and more solid. Finally, when Syme estimated they were about nine kilometers down, there was even a suggestion of moisture. The tunnel debouched at last into a large cavern. There was a phosphorescent gleam from fungus along the walls, but Syme couldn't decide how far away the far wall was. He noticed something else, though. "There's air here," he said to Tate. "I can see dust motes in it." He switched his helmet microphone from radio over to the audio membrane on the outside of the helmet. " "Yes, there is air here," said the Martian leader, startlingly. "Not enough for your use, however, so do not open your helmets." Syme swore amazedly. "I thought you said they didn't speak Terrestrial," Tate said. Syme was expressionless, but Syme sensed that he was amused. "Yes, you're "No," the Martian interrupted him, "revealing the extent of our The Martian looked at him appraisingly. "You already suspect. the Martians from knowing about it? Then he realized that the Martian know." Syme noticed that the other Martians had retired to the farther side of the cavern. Some were munching the glowing fungus. That left only the leader, who was standing alertly on all fours a short distance away from them, holding the Benson gun trained on them. Syme tried not to Mars to a planet with Terrestrially-thick atmosphere." at them with a queer intentness. "Well—how about the Martians—the Kal-Jmar Martians, I mean? I'd dearly love to know the answer to that "And yet," Tate mused, "you are being destroyed by contact with "We hope to win yet," the Martian said. sights of the Martian's Benson gun, and suddenly the pent-up rage in Martian. It was like tangling with a draft horse. The Martian was astonishingly tear it out of the Martian's fingers. And all the time he could almost feel the Martian's telepathic call for help surging out. He heard the swift pad of his followers coming across the cavern. He put everything he had into one mighty, murderous effort. Every muscle fiber in his superbly trained body crackled and surged with weapon and snapped a shot at the nearest Martian. The creature dropped dodging spears and other missiles in midair, and roaring at the top of his powerful lungs. Only then did he turn, to see Tate flattened against the wall behind Still silently, Syme refilled his oxygen tank, watched Tate do the

<question>:
How do the Martians detect Syme and Tate on the surface?

<options>:
A They have radar on the surface of Mars
B They patrol on foot
C It’s not revealed how they detect them
D They can sense rumbling from their underground caves

<answer>:

C