with a tank of air on your back ,you can

with a tank of air on your back ,you can
Skin-diving（轻装潜水）is a new sport today. This sport takes you into a wonderful world. It is like a visit to the moon. When you under water, it is easy for you to climb big rocks, because you are no longer heavy.
Here, under water, everything is blue and green. During the day, there is much light. When fish swim nearly, you can catch them with your hands.
When you have tanks of air on your back you can stay in deep water for a long time. But you must be careful when you dive in deep water.
To catch fish is one of the most interesting parts of this sport. Besides, there are more uses for skin-diving. You can clean ships without taking them out of water. You can get many things from the deep water.
Now you see skin-diving is both useful and interesting.
With a tank of air on your back, you can __________.
A. catch fish easily
B. stay under water for a long time
C. climb big rocks
D. have more fun
为什么选择D，不是B