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 andthe camel in the most water-deprived environments, and marine vertebrates in an all-waterenvironment, 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 standingwater is nonexistent, temperature is high, and humidity is low. Despite these handicaps, desert ratsare able to maintain the osmotic pressure of their blood, as well as their total boy-water content, atapproximately the same levels as other rats. One countermeasure is behavioral: these rats stay inburrows 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 evaporativecooling. Also, desert rats’ kidneys can excrete a urine having twice as high a salt content as seawater.
Marine vertebrates experience difficulty with their water balance because though there is noshortage 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 mostmarine vertebrates are unable to excrete a urine in which the salts are more concentrated than inseawater. Most of these animals have special salt-secreting organs outside the kidney that enablethem to eliminate excess salt.
Which of the following most accurately states the purpose of the passage？

A. To compare two different approaches to the study of homeostasis
B. To summarize the findings of several studies regarding organisms’ maintenance of internal variables in extreme environments
C. To argue for a particular hypothesis regarding various organisms’ conservation of water in desert environments
D. To cite examples of how homeostasis is achieved by various organisms
E. To defend a new theory regarding the maintenance of adeuate fluid balance