A、Too little sunlight. B、Survival competition. C、Severe heat. D、Too much rain. C原文提及“大多数动物需要保持体温的恒定，如果体温上升超过5摄氏度，它们就会死亡。”故C正确。

Some desert animals can survive the very strong summer heat and dryness because they have very unusual characteristics. The camel, for example, can bear an increase in the temperature of its body and its blood of 9℃ In addition, it can drink an enormous amount of water at one time, then store sufficient water in its red blood cells and other parts of its body to supply its need for two weeks or more. The kangaroo rat, on the other hand, obtains all the water it needs from water that it produces during respiration. However, most animals need to maintain a fairly constant body temperature, and will die if it rises more than 5℃ Therefore, they need to find some way to avoid the strong sun rays. Nor can many animals either store or produce water in their bodies, as the camel and kangaroo can. So they must find ways to reduce water loss from their bodies to the lowest degree.
    Because very few desert animals can survive the strong rays, the temperature, and the evaporation rate of a typical summer’s day, most of them are active during the night. Only after the sun has set does the desert come fully to life. The night is relatively cool, and the darkness provides protection, not only from the sun, but also from other animals and from the birds. So the coming of darkness is the signal for the large majority of animals and insects to continue their search for water and food. When morning comes, most of them seek shelter again.
16. What does the speaker say about the camel?
17. According to the speaker, in what condition will most animals die?
18. According to the speaker, why are so many animals active during summer night?