Three small fish lived in the sea. When they were very young, their parents died. They had to  15  each other day after day. Finally, they grew up and   16  to swim deeper and further.
One morning, the three fish were   17  to a shallow(浅) water by a big wave(浪). A small boat was taken there, too. It was hard for the fish to go back to the sea because the boat was right in front of them. The largest fish realized that they were   18  , and said they had to find a way to leave there as fast as possible. The second largest fish   19  with his elder brother, but the youngest one did not. He said that there was much   20  for their activities, so he would not worry at all and still swam around   21 .
At last, the largest fish returned to the   22  by jumping over the lowest part of the boat. The second largest fish also succeeded in   23   his elder brother in the other side of water. But the youngest one didn’t follow them and still swam joyfully.
Two hours later, when the sun rose up, the water in the small shallow corner   24 . The fish who did not try to return to the sea died of lack(缺乏) of water.

Three small fish lived in the sea. When they were very young, their parents died. They had to  15  each other day after day. Finally, they grew up and   16  to swim deeper and further.

One morning, the three fish were   17  to a shallow(浅) water by a big wave(浪). A small boat was taken there, too. It was hard for the fish to go back to the sea because the boat was right in front of them. The largest fish realized that they were   18  , and said they had to find a way to leave there as fast as possible. The second largest fish   19  with his elder brother, but the youngest one did not. He said that there was much   20  for their activities, so he would not worry at all and still swam around   21 .

At last, the largest fish returned to the   22  by jumping over the lowest part of the boat. The second largest fish also succeeded in   23   his elder brother in the other side of water. But the youngest one didn’t follow them and still swam joyfully.

Two hours later, when the sun rose up, the water in the small shallow corner   24 . The fish who did not try to return to the sea died of lack(缺乏) of water.

—Look，how happily the boys are swimming in the pool!

—How long ________ they ________?

—For about an hour．

A．have，been swimming                             B．have，swum

C．do，swim                                                 D．have，be swimming