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

<passage>:
understand it all. But—Martin, dying alone like that in a place like cautious advance of three awed space-scouters. An hour later they surfaced about two hundred yards away from the standing by the scoutships, and at the edge of the city, began to run toward them. "Radio picked you up as soon as you entered the pipe," someone said. It The men from the ship knew only the surface facts about this waiting this planet was still close enough to their own to permit a safe return in their ships, had not spotted the city. But the eleventh expedition Rodney's laughter rose, too. Then, sobering—"Maybe they crawled." A nebulous image, product of childhood's vivid imagination, moved slowly across Martin's mind. "All right!" he rapped out—and the image faded. "I imagine they had illumination of some sort," Martin answered, dryly. "If we don't hurry up and get through this place and back to the ship, "Martin!" Wass' voice came through the receivers in both their radios. "Martin, I can't get out!" it's like a glass wall." "We're trapped, we're trapped, they are—" Wass' voice sounded startled. "Anti-radiation screen?" Rodney interrupted, "There hasn't been enough radiation around here for hundreds of thousands of years to activate such a screen." and stood watching him labor along the metal toward them. Wass' face, Martin saw, was sober. "I tried to call the ship. No luck." "The shield?" Martin shook his head. "No. To be effective, the shield would have to downward, gleaming in the pale circle of his torch. A call from Rodney halted him. "Back here," the tall man repeated. "It looks like a switchboard." The three advanced to the end of the central corridor, pausing before a great arch, outlined in the too-careful geometrical figures Martin had come to associate with the city builders. The three torches, shining through the arch, picked out a bank of buttons, handles ... and a thick rope of cables which ran upward to vanish unexpectedly in the metal roof. "Is this it," Wass murmured, "or an auxiliary?" assumptions ever since we got here." "What would you suggest, instead?" Martin asked calmly. Rodney furtively, extended one hand toward a switch. "No!" Martin said, sharply. That was one assumption they dared not make. Rodney turned. "But—" switch again. Martin swore, stepped forward, pulled him back roughly. He directed his flashlight at Rodney's thin, pale face. "What do you think you're doing?" it, get ourselves aboard, and get the big ship away from the planet. Rodney cursed. "And every second we stand here doing nothing gives us that much less time to find a way out. Martin—" "Make one move toward that switchboard and I'll stop you where you stand!" Wass moved silently through the darkness beyond the torches. "We all Martin hung back waiting for the others to go ahead of him down the metal hall. At the other arch, where the ramp led downward, he called a halt. "If the dome, or whatever it is, is a radiation screen there must "In the pitch dark," Wass added. Martin adjusted his torch, began to lead the way down the metal ramp. Wass turned his light parallel with the floor, and then moved slowly in a circle. "No supports. No supports anywhere. What keeps all that up there?" three lights, past openings in the occasional high metal partitions, past something which was perhaps a conveyor belt, past another "We go back, I guess," Martin said. Rodney turned swiftly to face him. Martin thought the tall man was holding his gun. "To the switchboard, Martin?" "Unless someone has a better idea," Martin conceded. He waited. But Rodney was holding the gun ... and Wass was.... Then—"I can't think of matchstick thin, even in his cumbersome suit, swayed with jaunty triumph in the rear. But—The only city, so far as anyone knew, on the first planet they'd Torchlight reflected from Wass' faceplate as he turned his head. "Why not?" They were like children.... "We don't know, released, what they'll do." super-swiftness at the air valves and joints of their suits "—only the little moisture in the atmosphere." They stood before the switchboard again. Martin and Wass side by side, Rodney, still holding his gun, slightly to the rear. Rodney moved forward a little toward the switches. His breathing was Postpone and postpone until it's too late, and the ship leaves without us and we're stranded here to eat seeds and gradually dehydrate ourselves and God only knows what else and—" He reached out convulsively and yanked a switch. Martin leaped, knocking him to the floor. Rodney's gun skittered away silently, like a live thing, out of the range of the torches. The radio receivers impersonally recorded the grating sounds of Rodney's sobs. Martin glanced at Wass, then started out of the switchboard room, not looking back. "It got in and out of the city some way. Perhaps we can leave the same way." Down the ramp again. Rodney played his torch about the metal figures carefully placed about the floor. "Martin, what if there are no reservoirs? What if there are cemeteries instead? Or cold storage units? Maybe the switch I pulled—" "Rodney! Stop it!" "... The reservoirs, Wass. The pattern will still be here for later expeditions to study. So will we if we don't find a way to get out." Their radios recorded Rodney's gasp. Then—"Martin! Martin! I think I've found something!" easily—rotating the disk as it turned. Suddenly, without a sound, the disk rose, like a hatch, on a concealed hinge. The three men, clad in their suits and helmets, grouped around the six-foot opening, shining their torches down into the thing that drifted and eddied directly beneath them. Rodney's sudden grip on Martin's wrist nearly shattered the bone. "Martin! It's all alive! It's moving!" Martin hesitated long enough for a coil to move sinuously up toward the opening. Then he spun the wheel and the hatch slammed down. He was shaking. The hatch rose again at Martin's direction. He stood beside it, "How deep," Wass said, from his safe distance. of the hatch to stare coldly at Wass. "You'd rather monkey with the switches and blow yourself to smithereens?" Wass sighed and refused to meet Martin's gaze. Martin looked at him disgustedly, and then began to descend the rope, slowly, peering into "Not me," the answer came back quickly. "You two fools go your way, I'll go mine." "Wass!" There was no answer. The light faded swiftly away from the opening. The going was hard. The dust clung like honey to their feet, and eddied and swirled about them until the purifying systems in their suits were The two men lifted their feet out of the dust, and then laboriously plunged forward, to sink again to the knees, repeated the act, times without number. A grate. Rodney stared. "Wass!" he shouted. "We've found a way out!" Their radios recorded Wass' laughter. "I'm at the switchboard now, Martin. I—" There was a tinkle of breaking glass, breaking faceplate. The grate groaned upward and stopped. Wass babbled incoherently into the radio for a moment, and then he began to scream. Martin switched off his radio, sick. He turned it on again when they reached the opening in the metal wall. "Well?" "I've been trying to get you," Rodney said, frantically. "Why didn't Rodney said nothing. Then Martin said, "Did you listen until the end?" Rodney nodded, jerkily. "He pulled three more switches. I couldn't

<question>:
How did the crew discover the shield?

<options>:
A They went to the roof of the tallest building
B Wass tried to cross to retrieve forgotten equipment
C Martin and Rodney tried to move past the city's edge
D They activated it using the switchboard

<answer>:

B