A、To escape from their enemies. B、To catch more food. C、To use their developing flippers. D、To swim. B事实细节题 由于鲸鱼捕食的猎物是各种鱼类，久而久之，

Whales--or their ancestors--were once land animals. Scientists believe that they roamed the earth about 150 million years ago.
    The land-dwelling ancestors of this modem king of the sea were hunters with legs and the jaws and teeth of killers. Their favorite hunting grounds were probably shallow waters near the mouths of rivers or off a level stretch of coast, for fish, both finny and shell, were then more plentiful in the water.
    Ages passed, and it became easier for these creatures to swim than to walk; millions of years later they were able to do without legs altogether. Now their hind legs had shrunk so that mere traces of them can be found under the skin when a whale is dissected.