Homeostasis, an animal’s maintenance of certain internal variables within an acceptable range, particularly in extreme physical environments, has long interested biologists. The desert rat and the camel in the most water-deprived environments, and marine vertebrates in an all-water environment, encounter the same regulatory problem: maintaining adequate internal fluid balance.

For desert rats and camels, the problem is conservation of water in an environment where standing water is nonexistent, temperature is high, and humidity is low. Despite these handicaps, desert rats are able to maintain the osmotic pressure of their blood, as well as their total body-water content, at approximately the same levels as other rats. One countermeasure is behavioral: these rats stay in burrows during the hot part of the day, thus avoiding loss of fluid through panting or sweating, which are regulatory mechanisms for maintaining internal body temperature by evaporative cooling. Also, desert rats’ kidneys can excrete a urine having twice as high a salt content as sea water.

Camels, on the other hand, rely more on simple endurance. They cannot store water, and their reliance on an entirely unexceptional kidney results in a rate of water loss through renal function significantly higher than that of desert rats. As a result, camels must tolerate losses in body water of up to thirty percent of their body weight. Nevertheless, camels do rely on a special mechanism to keep water loss within a tolerable range: by seating and panting only when their body temperature exceeds that which would kill a human, they conserve internal water.

Marine vertebrates experience difficulty with their water balance because though there is no shortage of seawater to drink, they must drink a lot of it to maintain their internal fluid balance. But the excess salts from the seawater must be discharged somehow, and the kidneys of most marine vertebrates are unable to excrete a urine in which the salts are more concentrated than in seawater. Most of these animals have special salt-secreting organs outside the kidney that enable them to eliminate excess salt.

262. According to the passage, the camel maintains internal fluid balance in which of the following ways?

I. By behavioral avoidance of exposure to conditions that lead to fluid loss

II. By an ability to tolerate high body temperatures

III. By reliance on stored internal fluid supplies

(A) I only

(B) II only

(C) I and II only

(D) II and III only（B）

(E) I, II, and III

Internal water is not water? Since ETS indicats they conserve internal water, we can not eliminate III. So B is not complete answer. D is better.

Any comment is highly appreciated.

想问一下:

从文章得出:

1. Camel只有在温度很高的前提下, 才用sweating and panting来conserve internal water;--- special mechanism

2. sweating and panting又是降低interanl fluid level(internal water)的.

那么是否可以这么认为:

如果在极高温情况下, Camel不用sweating and panting, 就没办法来conserve interal water.

不知我的理解是否正确??

我看答案的时候也觉得这句很绕，不能说明问题，后来发现是自己把句子成分看错了

by（sweating and panting only when their body temperature exceeds that which would kill a human）, they conserve internal water.

这里的when从句是修饰sweating and panting的，就是说，它们是通过（只有在体温极高的情况下才排汗和呼吸）来保持体内水分的，而不是说在体温极高的情况下，它们通过排汗和呼吸来保持体内水分。

希望能给和我犯了同样错误的人一点帮助。