Both warm-blooded and cold-blooded desert animals have ways to escape the desert heat. Warm- blooded desert animals, such as rats and mice, rest during the day, often staying in cool underground burrows. At night they search for food. Animals that are out during the day, such as cold-blooded lizards and snakes, are active only for short periods. As their body temperature rises, these reptiles（爬行动物） move into the shade in order to cool down. In the early evening, when the sun grows weaker, the reptiles become more active and begin their search for food again.
　　Getting enough water to survive is a major problem for all desert animals. Some desert animals, like desert birds, manage to find water holes. Other desert animals, such as the kangaroo rat and the related jerboa, get water only from the food that they eat. Because these animals eat mainly dry seeds, they must survive on a tiny amount of water.
　　Most deserts have only a small number of frogs and toads because these animals must be near water to survive. Yet even these creatures have adapted(适应) to desert conditions. When small amounts of water collect in temporary streams, the desert-living frogs and toads become active. After a rainfall, they lay their eggs. The eggs grow into tadpoles in a few days and into adults in just four weeks. When the puddles(水坑) dry up, the adult frogs or toads dig into the ground. Their metabolism(新陈代谢) slows, and they stay beneath the ground until the next rain, which may be as good as a year away. Until then, their bodily activities continue at a reduced rate.
　　The camel — often called the ship of the desert — is one of the most successful desert animals. Camels can go for long periods without water, but eventually they must drink. When water becomes available to them after a long drought, they may drink 95 liters of water or more. When water is not available, what helps camels survive the desert heat is the fat stored in their humps(驼峰). A camel’s hump contains about 12 kilograms of fat. Fat is rich in hydrogen. As the fat is digested, hydrogen from the fat combines with oxygen in the air that the camel breathes. The result is H2O, or water. Each kilogram of fat that a camel digests produces about a liter of water.
16．Desert animals are usually more active at night because _____.
A．it is cooler at night         B．it is easier to find water
C．they like the dark           D．they are less likely to be attacked at night
17．Which of the following desert animals can get water only from the food?
A．The camel.       B．The kangaroo.        C．The frog.      D．The rat.
18．Which of the following is TRUE according to the passage?
A．All the desert animals rest during the day.
B．All the desert animals don’t rest during the day.
C．Cold-blooded desert animals are out most of the day.
D．None of the cold-blooded desert animals go out during the day.
19．The underlined word “burrows” in the first paragraph can be replaced by _____.
A．holes    B．caves         C．rooms              D．openings