Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 31 to 35.

LITTER ON MOUNT EVEREST

Mount Everest is the highest mountain in the world. It is also the highest trash pile in the world. About 400 people try to reach the top every year. Having many climbers means a lot of trash.

It is very dangerous to climb Mt. Everest. The air is very thin and cold. Most people carry bottles of oxygen; they could die without it. When the oxygen bottles are empty, people throw them on the ground. When strong winds rip their tents, people leave them behind. They don't have the energy to take the trash away. They only have enough energy to go down the mountain safely.

Trash is a terrible problem. Since people first began to climb Mt. Everest, they have left 50,000 kilos of trash on the mountain. Several groups have climbed the mountain just to pick up the trash. When people plan to climb the mountain, they have to plan to take away their trash.

Where do climbers put their empty oxygen bottles?