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

<passage>:
Brightside I learned to spot a passable crack, and tell it Crossing had no stomach for mysteries, vast or trifling, and there about the quiet lounge. Street trade was discouraged at the wisecracks, witticisms, unfunny jokes that wore thin now and then, never far, but a little further each time. the sky and hotter. Without our ultra-violet screens and glare end of an eight-hour trek. telecasts,” he said coldly. “The news was correct. We are going “At perihelion?” “Of course. When else?” and, for once, he didn’t have anything to say. It was the wreck middle, half-buried in a rock slide. A dozen feet away were where have you been hiding? gone. “Great balls of fire, man— their Brightside Crossing. felt different. I could see the dull gray stuff seeping up around the hubs, lead, steaming under a soft layer of concealing ash. an area of recent surface activity It was the planet that whipped us, that and the Sun broad flat plateau, dodging a network of gaping surface cracks—winding a deep crack. I think. I followed the news stories like a tri-V serial and then a long, narrow ledge of gray stuff that lay down across Twilight Lab up there and began to get Brightside into my for some years and hung onto the title after he gave up his commission. He was with Armstrong on Mars during his Service days, did a good deal of the original mapping and surveying for the Colony there. I first met him on Venus we spent five years together up there doing some of the nastiest exploring miles of Venus jungle. I liked him and I trusted him. old times he told me about the Vulcan business, and how he’d been out to see Sanderson and the Twilight Lab on Mercury, and how he preferred a hot trek to a cold one any day of the year—and then he wanted to know what I’d been doing since dangerous, too.” “What trip?” “Brightside of Mercury,” the Major said. I whistled cautiously. “At aphelion?” days later? No, thanks. I want the Brightside without any nonsense about it.” He leaned across me eagerly. “I want to make a Crossing at perihelion and I want to cross on the surface. If a man can do that, he’s got Mercury. Until then, nobody’s got Mercury. I want Mercury—but I’ll need help getting it.” I’d thought of it a thousand times and never dared consider it. Nobody had, since Wyatt and Carpenter disappeared. Mercury turns on its axis in the same time that it wheels around the Sun, which means that the Brightside is always facing in. That makes the Brightside of Mercury at perihelion the hottest place in the Solar System, with one single exception: the surface of the Sun itself. how hellish and they never came back to tell about it. It would cross it. I wanted to be along. The Twilight Lab, near the northern pole of Mercury, was the obvious jumping-off place. The setup there wasn’t very extensive—a rocket landing, the labs and quarters for Sanderson’s crew sunk deep into the crust, and the tower that housed Twilight Lab wasn’t particularly interested in the Brightside, Mercury as the closest chunk of rock to the Sun that could hold his observatory. He’d chosen a good location, too. On Mercury, the Brightside temperature hits 770° F. at perihelion and the Darkside runs pretty constant at -410° F. No permanent installation with a human crew could survive at either extreme. But with Mercury’s wobble, the twilight zone between Brightside and Darkside offers something closer to survival temperatures. Sanderson built the Lab up near the pole, where the zone is about five miles wide, so the temperature only varies 50 to 60 degrees with the libration. The Solar ’scope could take that for about seventy out of the eighty-eight days it takes the planet to wheel around. The Major was counting on Sanderson knowing something about Mercury as well as the Sun when we camped at the Lab to make final preparations. Sanderson did. He thought we’d lost our minds and he said was like. liable to get awfully uneasy and none of them can ever give you an answer that makes sense. Anyway, Stone had borrowed three men from the Lab, and had the supplies and equipment all lined up when we got there, ready to check and test. “Not exactly. But Brightside is no place to count on luck.” far out that the detail dissolved into blurs on blow-up. They of our course. down toward the equator—not only volcanic, but sub-surface doubt of it. But the only way to avoid it is to travel over the less activity to the west. Now we might avoid some if we could localized. well. There was an atmosphere and a constant atmospheric flow from Brightside to Darkside. Not much—the lighter gases had reached escape velocity and disappeared from Brightside 2 , and nitrogen, and traces of other heavier gases. There was also an abundance of sulfur vapor, as well as carbon disulfide and sulfur dioxide. The atmospheric tide moved toward the Darkside, where it we would find out what was happening where was to be there. gray, and the sort of eyes that looked like a climber’s—half-closed, sleepy, almost indolent, but capable of abrupt alertness. Stone and McIvers were thick as thieves, and everything was them. “There are lots of bizarre personalities around a place like this, and some of the best wouldn’t seem to be the most the two layers. Warning thermocouples, of course—at 770 degrees, it wouldn’t take much time to fry us to cinders water and zinc was almost at melting point and the pools of We jumped off from Twilight on a course due southeast with thirty days to make it to the Center of Brightside. If we could cross an average of seventy miles a day, we could hit Center exactly at perihelion, the point of Mercury’s closest approach to the Sun—which made Center the hottest part of the planet at the hottest it ever gets. The Sun was already huge and yellow over the horizon two thousand miles to the opposite twilight zone. Sanderson “It sounds like I am,” I cut in. “We want to keep the lead He stared at the Major. “I mean, how can we tell what sort of “Charts! I’m talking about detail I can stay on good solid ground, of course, but scan the never forget that huge yellow Sun glaring down, without a break, hotter and hotter with every mile. We knew that the the Twilight Lab. I moved out first back over my shoulder, I could see the of Brightside. But in a couple of hours we rumbled past Sanderson’s were in virgin territory and already the Sun was beginning to degrees Fahrenheit inside our suits, but our eyes watched that glaring Sun and the baked yellow rocks going past, and sweat as if we were in a superheated furnace. The sun-shield cut the temperature down sixty or seventy We didn’t sleep the first few stops, as a consequence. Our the wheel. We were moving down into desolation that made Earth’s old Death Valley look like a Japanese rose garden. with a barely visible yellowish mist of sulfur and sulfurous gases. It was a hot, barren hole, no place for any man to go, but worse heat in the Solar System was the surface of the Sun get us. That was the bargain. I learned a lot about Mercury those first few driving periods. The gorge petered out after a hundred miles and we moved onto the slope of a range of ragged craters that ran south and east. This range had shown no activity since the first landing on Mercury forty years before, but beyond it there were active cones. Yellow fumes rose from the craters constantly their rubble. Below were the vast yellow flatlands, smoking and hissing from the gases beneath the crust. Over everything was gray ash, filling crevices and declivities—offering a soft, treacherous

<question>:
What is the twilight zone?

<options>:
A The place at the end of the Brightside Crossing.
B A lab where they study the Sun.
C The place between Brightside and Darkside.
D A lab where they study Mercury.

<answer>:

C