阅读下面短文，掌握其大意。然后从各小题所给的A、B、C、D四个选项中选出最佳选项。

   Do you like dolphins(海豚)? Dolphins live1water, but they are not 2. They are mammals (哺乳动物) that live in water. It has to(不得不) breathe(呼吸) air to the top of the water very often. A dolphin's nose isn't see. It's just a very small hole(孔)3top of the dolphin's head. This hole is called a blowhole. A dolphin comes up(出现) every minute or so to breathe. It4blowhole and takes air in, and then goes back into the water again. It can stay up the5for six or seven minutes if it has to. A dolphin's ears are very small,6its mouth is very 7.It has almost one hundred teeth. A dolphin8very fast. Scientists are studying ways to9boat go faster through the 10 .