单项选择题

Mountain climbers around the world dream about going up Mount Everest (珠穆朗玛峰). It is the highest mountain in the world. But many people who have climbed the mountain have left waste material that is harming the environment (环境).
A team of Americans is planning the largest clean-up effort ever on Mount Everest. They will make the risky trip up the mountain next month. The team is made up of nine Americans. Their goal is to remove all the trash (垃圾) they see. They will spend two months cleaning up the mountain by gathering all kinds of trash. They are expected to remove at least three tons of trash in large bags.
Team leader Doug Bell is making his fourth trip up the mountain. He says he hopes to bring Everest to the condition it was in before the first successful climb fifty years ago. He says he hopes the effort will influence (影响) other people to clean up the environment closer home. Human waste on Everest is a major concern (关注的问题). So the clean-up team will take along newly developed equipment to collect and treat human waste. Over the years, the waste articles have polluted the mountain. In the warm season when the ice melts, the polluted water flows to Nepali villages below. The problem has gotten worse in recent years because climbing Everest has become more popular. When those Americans are planning the trip up Mount Everest, what are they concerned about