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

<passage>:
that kept his body hard and resilient in spite of long hours spent in the laboratory. As he opened the door of his apartment he heard the awake. It was the door that was being attacked, not his head. The and reached for his bathrobe. It wasn't there. He swore some more and grabbed a towel, wrapping it inadequately around him it didn't quite meet astern. He paddled wetly across the floor sounding like a flock of ducks on parade. A hammering thunder burst on the outer door. He sat up with a groan. "Lay off, you crazy apes!" he yelled furiously, but the pounding "Come here, you idiot!" Brian screamed after his erstwhile assistant. rattled the bars. "Hello, pantless!" a voice breathed. He knew that voice! They burst around a corner onto a startled guard. The electric eye tripped a screaming alarm, but the broken key in Brian's hands opened the complicated lock in a matter of seconds. They jammed immovably behind them. Brian felt as though his stomach had fallen down around his ankles bomb thundered along the street and slammed them to the ground. The roar. The chase wasn't organized yet, and they soon lost themselves in the orderly rush of Venus City traffic. stabbed through the darkness and they clambered precariously down a "Old mines? What old mines?" Brian was startled at the icy hardness of her voice. mines. A brand-new atomic motor gleamed incongruously at one end. The rebels crowded into it and they went rumbling swiftly down the echoing The tunnel emerged in a huge cavern that gloomed darkly away in every to be swallowed to a whisper in the vast, echoing darkness. Between two rows of sentinel ruins they came suddenly on two slim Venusian atmospheric ships. Dim light spilled over them from a ragged gash in the wall of the cavern. Brian followed Crystal into the smaller "Through that hole up there," the girl said matter-of-factly. "You're crazy, you can't get through there." "Oh, yeah? Just watch this." The ship thundered to life beneath them and leaped off in a full-throttled take-off. "We're going to crash! That gap isn't wide enough!" set in grim lines as she pulled the ship up in a screaming climb. Brian got up off the floor. "You don't have to get excited like that," he complained. "They weren't "You must be mistak—" He slumped to the floor as Crystal threw the ship into a mad, rolling spin. A tremendous crash thundered close astern. "I guess that was a mistake!" Crystal yelled as she fought the controls. streak of paint and metal ripped off the wing of the police ship. He saw the crew battling their controls in startled terror. The ship slipped frantically away and fell into a spin. jerked and jumped like crazy marionettes as the bullets smashed into through the fuselage. "They're after us now!" Crystal's answer was to yank the ship into a rocketing climb. The police were watching for that. The big ship roared up after them. She snapped the ship into a whip stall. For one nauseating moment they hung on nothing, then the ship fell over on its back and they screamed down in a terminal velocity dive, heading for the safety of the lower " Brian gasped. "Well, we got away that time. How in thunder the huge bulk of a mountain which blocked the entire width of the it up and up— "Look! Police ships. They've seen us!" The ship lunged straight for the mountain wall! "Are you crazy? Watch out—we'll crash!" of the mountain face. Brian yelped and cowered instinctively back. The through the foliage—there was no crash. They burst through into a huge, brilliantly lighted cavern and settled to a perfect landing. Men came running. Crystal tumbled out of her ship. "Who was that crazy coot and what is this place?" Brian demanded. thundered to life, streaked across the floor and burst out through the spotted for sure, now." The other rebels waited uncertainly, but not for long. There was the crescendoing roar of ships in a dive followed by the terrific crash of an explosion. "They got him!" Crystal's voice was a moan. "Oh, the fool, the fool!" "Sounded like more than one ship. They'll be after us, now. Is there any other way of getting out of this place?" "Not for ships. We'll have to walk and they'll follow us." around so their rocket exhausts sweep the entrance to the cavern," They ran two ships out into the middle of the cavern, gunned them cautiously into the cavern, guns ready, expecting resistance. They met a dead silence. A score or more followed them without any attempt at concealment. Then Brian and Crystal cut loose with the drives of the two ships. Startled screams of agony burst from the crowded group of police as they were caught in the annihilating cross fire of roaring flame. They crisped and twisted, cooked to scorched horrors before they fell. A burst of thick, greasy smoke rushed out of the cavern. Two of climbed blindly from her ship. "Let's get away! I can smell them burning," she shuddered and covered "You don't have to. Wait here." He climbed back into one of the ships and cut the richness of the fuel mixture down till the exhaust was a lambent, shuddering stutter, verging on extinction. He dashed to the other ship and repeated the maneuver, fussing with the throttle till he had the fuel mixture adjusted to critical fineness. The beat of the stuttering exhaust seemed to catch up to the other and built to an aching pulsation. In a moment the whole mass of air in the cavern hit the frequency with a subtle, intangible thunder of vibration. Crystal screamed. "Brian! There's more police cutting in around the entrance." Brian clambered out of the ship and glanced at the glowing points in the rock where the police were cutting their way through outside the line of the exhaust flames. The pulsating thunder in the cavern crescendoed to an intolerable pitch. A huge mass of stalactites crashed to the floor. Crystal led the way as they fled down the escape tunnel. The roaring crash of falling rock was a continuous, increasing avalanche of sound in the cavern behind them. They emerged from the tunnel on the face of the mountain, several hundred yards to the east of the cavern entrance. The ground shook and heaved beneath them. "The whole side of the mountain's sliding," Crystal screamed. "Run!" Brian shoved her and they plunged madly through the thick tangle Huge boulders leaped and smashed through the matted bush around them. Crystal went down as the ground slipped from under her. Brian grabbed her and a tree at the same time. The tree leaned and crashed down the slope, the whole jungle muttered and groaned and came to life as it joined the roaring rush of the slide. They were tumbled irresistibly it stilled and left them bruised and shaken in a tangle of torn vegetation. The remains of two police ships, caught without warning in the rush as they attempted to land, stuck up grotesquely out of the foot of the slide. The dust was settling away. A flock of brilliant blue, gliding Brian and Crystal struggled painfully to solid ground. Crystal gazed with a feeling of awe at the devastated mountainside. "How did you do it?" "It's a matter of harmonics," Brian explained. "If you hit the right vibratory combination, you can shake anything down. But now that we've made a mess of the old homestead, what do we do?" scrambling through the jungle up the mountainside.

<question>:
What caused the avalanche of rocks in the cavern?

<options>:
A An atomite bomb.
B A chemical reaction with the ships’ exhaust.
C The vibrations of the advancing police.
D Vibrations from the ships’ exhaust.

<answer>:

D