D. It makes them attractive to fish.
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
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.
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.
Passage 6
Questions 20 to 21 are based on the following passage:
Since 1960 a new kind of tool has appeared. This is the silicon chip -- a little chip of silicon crystal. It is smaller than a finger nail, but it can store more than a million "bits" of information. It is an electronic brain.