单选题 A question often put to the specialist on fishes is "How long do fishes live?" This puts the specialist in an embarrassing position because he is often unable to give a direct answer to this simple question.
But actually this question is not as simple as it seems. There are thousands of different kinds of fishes, and they vary a great deal in size and life span. Moreover, it is not easy to find out just how long a fish lives in its natural state.
We can find out how old a fish is by studying its scales, but we cannot say how much more time it would live if we had not caught it.
We may rear fishes and record their life span but we cannot be sure that this is the length of time they would have lived, had they been left alone.
We may make marking to show how fast the fishes grow so that we can calculate the age of the largest on record, but unless this large fish dies of old age we are still not in a position to know its natural life span.
Unlike human beings, fishes do not stop growing when they reach maturity (成熟). They continue to grow as long as they live, although the rate of growth slows down in mature fishes.

单选题 People often ask the specialist on fishes regarding its ______.

单选题 The specialist is embarrassed by the question because ______.

单选题 We can know a fish"s age from its ______.

单选题 Which of the following statements is NOT true?

单选题 Human beings, unlike fishes, ______.