The word "smart" in the last paragraph means __________.

A．A dolphin can stay under the water for about six minutes.
B．A dolphin often comes to the top of the water to take in air.
C．Dolphins are not fish but mammals in the sea.
D．A dolphin doesn't have a good hearing.

A．to learn how fast it can swim
B．to discover the value of the dolphin
C．because dolphins are good swimmers
D．because they like swimming very much