For a long time, mountains were thought to be file home of evil spirits(幽灵). People believed dragons lived on the mountainside. They thought these dragons would gobble up anyone coming near.
When men began to climb mountains, they faced many dangers. The air high on a mountain was freezing cold. There were deep cracks (裂缝)into which a climber could fall. Roaring winds and terrible storms could sweep a man off the mountainside.
At first, men did not have the proper equipment to climb high mountains. They did not have ice axes, ropes, or the right kind of clothing.
Later, men could not reach the top of the highest peaks for another reason. As a man climbed higher, the air became thinner. There was less oxygen to breathe. Above 1800 feet, a climber had to stop every few steps to catch his breath. His heart beat faster and his blood became thicker. Above 25, 000 feet, a climber would begin to see things that were /tot there. He could no longer think clearly about what he had to do.
This passage mainly tells us about _______.

A. the dangers of climbing high mountains
B. the evil spirits and dragons that live on mountains
C. the proper equipment needed to climb mountains
D. people who live in mountains