题目内容：
Questions are based on the following passage.Salt is as necessary to life as water. In many areas of Africa people once drank the urine (尿）
of animals because they had no other source of salt. Without salt, human beings die.
The human body demands that the amount of salt in the blood always stay the same. When the body does not get enough salt, it protects itself by letting less salt leave the body in urine and sweat. But it carmot reduce this output to zero. Some salt is always escaping. On a complete saltless diet, like that of some people in Africa, the body steadily loses small amounts of salt through the kidneys (肾) and sweat glands (腺）It then tries to adjust to this loss by speeding its secretion (分泌) of water. (78) In this way, the body attempts to keep the amount of salt in the blood at the necessary level. The result is a slow drying up of body and,inally,death. The person dies of thirst.
In cases where there is little or no water to drink, the body tries to do the opposite thing. Again, it must keep the salt level in the blood constant. Because it has little water, it attempts to stop water from leaving the body and to increase its secretion of salt. But, as with the escaping salt, it cannot be completely successful. Some water still leaves and the person eventually dies of thirst. In short, the body's normal needs for salt and for water are both parts of the same important need to keep the salt level in the blood constant.
Some African people once drank animal urine __________ A．when they were going to die
B．because they were thirsty
C．because there was little salt
D．because they were short of water
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