We are used to the idea of aging in ourselves. We are so used to this that it comes as a surprise to find that there may be some animals that do not age. Sea anemones(海葵) are an example. Some have been kept for nearly a century without showing any signs of lifelessness. Some kinds of sea worms can even “grow backwards.” If kept in the dark and given nothing to eat, they get steadily smaller, They finally end as a ball of cells(细胞) looking rather like the egg from which they came. Under good conditions the ball will turn back to a worm and start growing again. One could probably keep them growing and un-growing again and again. Some sea worms grow smaller when they ______.

A.
lose weight

B.
live in the darkness

C.
are under good conditions

D.
don?t eat and are kept in the dark

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