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

<passage>:
said. "Some of those legs it stands on, transmission belts, blastoff. There was nothing to do but the stubbornest Dutchman on Venus. to prove it, but on Venus there was in Rathole were able to build these semi-underground domes to resist the earth shocks that came howling gale that swept from the west. The huge wheels twisted and jolted against the rocks outside, What on Venus could have happened and Jan bounced against his from Den Hoorn. But this one seat belt, wrestled the steering across the tumbled terrain of filled the airtight groundcar. There came a new swaying that was not the roughness of the terrain. Through the thick windshield Jan saw all the ground about him buckle and heave for a second or two before it settled to rugged quiescence groundcars out to search for it. This, of course, would be too here. We have no engines except groundcar radio. "That's the third time in half the stationary generators that Venus, and Heemskerk hoped vehicles, so our vehicles are operated by hand." "You push them?" demanded Jan incredulously. "No. You've seen pictures of the pump-cars that once were used on terrestrial railroads? Ours are powered like that, but we cannot operate them when the Venerian wind is blowing. By the time I diagnosed the Venus Shadow count before blastoff would start with a broom at his masthead! It was a high thin whine borne of brooms on Venus. over the area loudspeakers. reaction, considering the scarcity on the wind, carrying even through the walls of his spacehelmet, ramp. Around him, the rocket mechanics were staring up at the sky, trying to pinpoint the noise. Oostpoort in the northern hemisphere of Venus, while Heemskerk above the mountain, against the indigo umbrella of clouds. It only when the two planets are at the right position in their orbits. from its deck in front of them. of the Venus Shadow?" A broom? No. The platform hovered and him. As the platform descended to a few meters above ground, the cut ends of belts surface of Venus and return to a whipped out wildly and the platform slid to the ground with a rush. It hit with a clatter and its the ground. "Jan!" boomed Heemskerk, two passengers tumbled prone to forcing his voice through the helmet the typical mottled texture of the the roach-flat groundcar right, but I had no power to run them. So I had to pull the engines and rig up a power source." tripod of battered metal pipes, atop which a big four-blade propeller spun slowly in what wind Venus Company isn't exactly a base, hung a series of tattered transmission belts. "Power source?" repeated Heemskerk. "That?" He's suffering from the Venus Shadow." the groundcar into the teeth of the Twilight Gale. buck this wind," he said thoughtfully, "or I'd have come in one of a thin fault area of a planet whose crust was peculiarly subject to earthquakes, particularly at the beginning and end of each long day when temperatures of rough road through the ravines. But even the 2½-meter wheels of the groundcar had trouble amid over by a gust if not handled just right. The three earthshocks that had surface, or steep breaks where the crust had been raised or dropped several meters by past quakes. The groundcar zig-zagged slowly westward. The tattered violet-and-indigo clouds boiled crack that made further progress seem impossible. A hundred land. After a moment's hesitation, ring-shaped base was less than a meter thick, standing on four his passage. He drove on. Apparently it was the result of an old quake. He found a spot where rocks had tumbled down, making a steep, rough ramp up He made it just in time. He had driven less than three hundred meters when a quake more severe than any of the others struck. Suddenly behind him the which, like the rest of the structures in Rathole, was a half-buried dome. The platform's have to climb a cliff of equal height returning eastward. The ground heaved and buckled like a tempestuous sea. Rocks rolled and leaped through the air, several large ones striking the groundcar with ominous force. The car staggered forward on its giant wheels like a drunken man. The quake was so violent that at one time the vehicle was hurled several meters spotted with rust, Jan, surrounded And the wind smashed down on it unrelentingly. The quake lasted for several minutes, during which Jan was able to make no progress at all and struggled only to keep the groundcar upright. Then, in unison, both earthquake and wind died to absolute quiescence. Jan made use of this calm to step down on the accelerator and send the groundcar speeding forward. The terrain was easier transmitted to it by heavy rubber belts. sideways, and almost overturned. with more than a dozen windmills. Windmills! Tears came to the masthead." X minus twelve. In two minutes, of the tulip fields. darkening west, was spotted Amid the windmills, low domes protruded from the earth, indicating that the dwellings of Rathole the platform and electric motors to turn the propellers. But batteries big enough to power it all so heavy the machine couldn't lift against the wind. The savage gale piled dust and sand in drifts against the domes, then, shifting slightly, swept them them off the ground. If there chasm out on Den Hoorn. The groundcar! Jan straightened and snapped his fingers. "Doctor!" he explained. "Send a couple of men to drain the rest of the fuel from my groundcar. And let's get this platform above ground and tie it down until we can get it started." Sanchez gave rapid orders in run, carrying five-gallon cans with them. platform and carried it up a ramp and outside. As soon as they reached ground level, the wind hit them. They dropped the platform to the ground, where it shuddered and swayed momentarily, and two of the men fell successfully on their stomachs. The wind caught the third and somersaulted him half a dozen Venus was beginning to affect ground level, surveyed the terrain. There was flat ground to before any of the domes protruded and secured it by them to the heavy chain that ran by the dome. The platform quivered and shuddered in the heavy wind, but its base was too low for it to overturn. Shortly the two men returned with the fuel from the groundcar, struggling along the chain. the carburetors and spun the engines. Nothing happened. He turned the engines over again. One of them coughed, and removing his venushelmet. fuel and the platform must have high octane gasoline." Jan threw up his hands and Diego! " alert, but the mark of the Venus Shadow was on his face. There groundcar." Sanchez called orders to the men at the platform. While they done," he repeated. "We can't make the trip overland because of the chasm out there in Den groundcar. There was a score of men there now, and a few women. They let the pair go darker. best, but Venus had beaten him. quake.... The ground of Den Hoorn was still shivering. Jan did not realize this until he had to brake the groundcar almost to a stop at one point, because it was not shaking in severe, periodic shocks as it had earlier. It quivered constantly, like the surface of quicksand. The ground far ahead of him the monotony of Venus that had and hit the brakes so hard the groundcar would have stood on its nose had its wheels been smaller. They skidded to a stop. The chasm that had caused him such a long detour before quake that had hit earlier. Now it was a canyon, half a kilometer wide. Five meters from the edge the bottom. Cursing choice Dutch profanity, Jan wheeled the groundcar

<question>:
What is the Venus Shadow?

<options>:
A The time of day when travel is impossible because there is no light.
B The name of Jan's aircraft.
C A deadly disease that can only be cured by traveling to Earth.
D The quakes that shake the ground.

<answer>:

C