It is one thing to land science instruments on Mars. However, it is quite 76.______ another to establish a base for’ humans to explore planet. Daytime temperatures 77.______ can be rise above freezing, but, because of the extremely thin atmosphere, 78.______ the sun heat radiates back into space. Even at the equator, the temperature 79.______ drops -50℃at night. In fact, there is no ozone (臭氧) layer to keep out 80.______ ultraviolet (紫外线的) radiation, and hardly some oxygen for either breathing 81.______ and burning conventional fuels. But despite all these problems, scientists are 82.______ currently working transport and clothing for Mars and an artificial environment 83.______ in which colonists could live. However, the potential cost make the idea of 84.______ human life on Mars nothing other than a fantastic dream. 85.______