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

<passage>:
fever, talking cockatoos and flagpole trees, I was running in circles—especially since Grannie became twins every now and then. it at its narrowest spot." Five minutes after my ship had blasted down, the four of us met in the offices of Interstellar Voice here at all is because of the presence of an ore known as Acoustix. Then it was found that this ore, Acoustix, revitalized their sounding apparatus, and the rush was on." "What do you mean?" "But up until now this moon is the only place where it can be found. "What sort of trouble?" Grannie Annie had demanded. And when Antlers Park stuttered evasively, the old lady snorted, "Fiddlesticks, I never saw trouble yet that couldn't be explained. We leave in an hour." But it wasn't a mirage. As the party came closer, we could see that Steadily the four travelers approached. Then, when a dozen yards away, they suddenly faded like a negative exposed to light and disappeared. "What do you make of it?" I said in a hushed voice. For some time I had fixed my gaze on a dot in the sky which I supposed slanted up into the sky to connect with the kite. goggles could not conceal. anybody can help me, you can." Grannie's eyes glittered. "Trouble with the mine laborers?" she questioned. car's ability to move in any direction. excellent living conditions with a vacation on Callisto every year. Up until a short time ago most of them were in excellent health and spirits. Then the Red Spot Fever got them." "Red Spot Fever?" Grannie looked at him curiously. Jimmy Baker nodded. "The first symptoms are a tendency to garrulousness on the part of the patient. Then they disappear." He paused to make an adjustment of the windlass. "They walk out into the Baldric," he continued, "and nothing can stop them. We tried following them, of course, but it was no go. As soon as they realize they're being followed, they stop. But the moment our eyes are turned, they give us the slip." a door in the central building opened, and a man emerged. His face was drawn. "Mr. Baker," he said breathlessly, "seventy-five workers at Shaft Four have headed out into the Baldric." Baker dropped his cigarette and ground his heel on it savagely. men away until the plague has died down?" Baker shook his head. "Three doctors from Callisto were here last month. They were as much at loss as I am. As for sending the men away, I may have to do that, but when I do, it means quits. Our company is chartered with Spacolonial, and you know what that means. Failure to produce during a period of thirty days or more, and you lose all rights." threw off the switch. "The entire crew of Shaft Four have gone out into the Baldric," he said slowly. There was a large map hanging on the wall back of Baker's desk. Grannie Annie walked across to it and began to study its markings. "Shaft Four is at the outer edge of the Baldric at a point where that level where a huge laboratory and experimental shop ran the length of the building. Grannie seized a light weight carry-case and began dropping articles into it. A pontocated glass lens, three or four roll down the ramp. Not until they had vanished in the desert haze did I sense the loneliness of this outpost. With that loneliness came a sudden sense of , and through glass doors I could see clerks busy with counting machines and report tapes. In another chamber the extremely light Acoustix ore was It was a scene of a rapidly unfolding desert country as seen from the entered the room. It stopped abruptly. "The machine uses a lot of power," the operator said, "and as yet we haven't got much." The cloud of anxiety which had wrapped itself about me disappeared Miss Flowers there?" "Miss Flowers left with Mr. Baker for Shaft Four," I said. "There's trouble up there. Red spot fever." "Fever, eh?" repeated Park. "That's a shame. Is there anything I can do?" "Tell me," I said, "has your company had any trouble with this plague?" "A little. But up until yesterday the fever's been confined to the group of Earth people suddenly materialized on the eyrie, talking and moving about as if it were the most natural thing in the world. powerful photographic impression of the object. That impression is foci. That eyrie might be likened to a cinema screen, receiving brain Then abruptly the screen before me blurred and went blank. "Sorry," the operator said. "I've used too much power already. Have to give the generators a chance to build it up again." Nodding, I turned and motioned to Karn. We went back downstairs. "That explains something at any rate," the old prospector said. "But how about that Red spot fever?" On Jimmy Baker's desk was a large file marked: FEVER VICTIMS. I opened it and found it contained the case histories of those men who had been attacked by the strange malady. Reading them over, I was struck by one detail. Each patient had received the first symptoms, not while working in the mines, but while sleeping or lounging in the barracks. Five minutes later Karn and I were striding down a white ramp that led to the nearest barracks. The building came into sight, a low rectangular structure, dome-roofed to withstand the violent winds. Inside double tiers of bunks stretched along either wall. In those bunks some thirty men lay sleeping. The far wall was taken up by a huge window of denvo-quartz. As I stood there, something suddenly caught Ezra Karn's eye. He began to walk "Look here," he said. metal had been wedged into an aperture cut in the quartz. The central part of the button appeared to be a powerful lens of some kind, and as I seized it and pulled it loose, I felt the hum of tiny clock work. All at once I had it! Red spot fever. Heat fever from the infra-red rays of Jupiter's great spot. Someone had constructed this lens to concentrate and amplify the power of those rays. The internal clockwork served a double purpose. It opened a shutter, and it rotated the lens slowly so that it played for a time on each of the sleeping men. I slid the metal button in my pocket and left the barracks at a run. Back in the visiscreen room, I snapped to the operator: "Turn it on!" The manager of Interstellar Voice turned again, and something glinted "What have you done with Miss Flowers?" I demanded. The manager's eyes glittered with fear as he saw my finger tense on the trigger. Weakly he lifted an arm and pointed to the northwest. I headed for that entrance, and when I reached it, I shut off power with an exclamation of astonishment. close. Ten feet forward, I stopped short, staring in amazement. Advancing toward me like a column of infantry came a long line of Larynx miners. They walked slowly, looking straight ahead, moving down the center of the gorge toward the entrance. had been removed, and mounted on the hood was a large bullet-like contrivance that looked not unlike a search lamp. A blinding shaft of vibratory scale and the only thing that will combat the infra-red rays that cause red spot fever. Those men won't stop walking until they've reached Shaft Four." Grannie Annie told her story during the long ride back to Shaft Four. It was he or his agents who placed those lens buttons in the Larynx to head her off before she reached Shaft Four. He did head her off and managed to lure her and Baker and Xartal into the Shaft barracks where they would be exposed to the rays from the lens button. But Grannie only pretended to contract the plague.

<question>:
What gives away the location of the lens?

<options>:
A When Billy enters the barracks, he realizes he's being hit by Red Fever.
B Workers were showing their first symptoms from working in the mines.
C The location was written in the Fever Victims file.
D Workers were showing their first symptoms after being in the barracks.

<answer>:

D