Mountain climbers around the world dream about going up Mountain 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. A team of Americans is planning the largest clean-up effort ever on Mountain Everest. They will make the risky trip up the mountain next month. The team of eight Americans will be guided by more than twenty Sherpas of Nepal (尼泊尔夏尔巴人).Their goal is to remove all the trash they see. They will spend two months cleaning up the mountain by gathering oxygen bottles, fuel containers, batteries, drink cans, human waste and other kinds of trash. They are expected to remove at least three tons of trash in large bags. Team leader Robert Hoffman 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 other people to clean up the environment closer home. Human waste on Everest is a major concern. So the clean-up team will take along newly development 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 Mountain Everest, they are concerned about______.