Passage 5
Questions 16 to 19 are based on the following passage：
Certainly no creature in the sea is odder than the common sea cucumber. All living creature， especially human beings， have their peculiarities， but everything about the little sea cucumber seems unusual. What else can be said about a bizarre animal that， among other eccentricities， eats mud， feeds almost continuously day and night but can live without eating for long periods， and can be poisonous but is considered supremely edible by gourmets?
For some fifty million years， despite all its eccentricities， the sea cucumber has subsisted on its diet of mud. It is adaptable enough to live attached to rocks by its tube feet， under rocks in shallow water， or on the surface of mud flats. Common in cool water on both Atlantic and Pacific shores， it has the ability to suck up mud or sand and digest whatever nutrients are present.
Sea cucumbers come in a variety of colors， ranging from black to reddish brown to sand color and nearly white. One form even has vivid purple tentacles. Usually the creatures are cucumber shaped —— bhence their name —— and because they are typically rock inhabitants， this shape， combined with flexibility， enables them to squeeze into crevices where they are safe from predators and ocean currents.
Although they have voracious appetites， eating day and night， sea cucumbers have the capacity to become quiescent and live at a low metabolic rate feeding sparingly or not at all for long periods， so that the marine organisms that provide their food have a chance to multiply. If it were not for this faculty， they would devour all the food available in a short time and would probably starve themselves out of existence.
But the most spectacular thing about the sea cucumber is the way it defends itself. Its major enemies are fish and crabs， when attacked， it squirts all its internal organs into water. It also casts off attached structures such as tentacles. The sea cucumber will eviscerate and regenerate itself if it is attacked or even touched; it will do the same if the surrounding water temperature is too high or if the water becomes too polluted.
16. According to the passage， why is the shape of sea cucumbers important?
A. It helps them to digest their food.
B. It helps them to protect themselves from danger.
C. It makes it easier for them to move through the mud.
D. It makes them attractive to fish.
正确答案是
17. The fourth paragraph of the passage primarily discusses ______.
A. the reproduction of sea cucumbers
B. the food sources of sea cucumbers
C. the eating habits of sea cucumbers
D. threats to sea cucumbers‘ existence
正确答案是
18. What can be inferred about the defense mechanisms of the sea cucumber?
A. They are very sensitive to surrounding stimuli.
B. They are almost useless.
C. They require group cooperation.
D. They are similar to those of most sea creatures.
正确答案是
19. Which of the following would NOT cause a sea cucumber to release its internal organs into the water?
A. A touch.
B. Food.
C. Unusually warm water.
D. Pollution.
正确答案是