补全对话

Peter:Hi, Sally. Come and have a look at my photos.

Sally:Are these the photos you took in Africa, Peter?

Peter:    1     Look, this is a wildebeest. It's an animal in Africa.

Sally:    2    

Peter:No, it doesn't. It only eats grass.

Sally:Are there a lot of animals in Africa?

Peter:   3    

Sally:Oh, this photo looks interesting. What are you doing there?

Peter:    4    

Sally:Did you have a good time there?

Peter:Ofcourse.    5    

工人师傅用如图所示的滑轮组提升重物，在大小为500N的拉力F作用下，10s内将800N的重物匀速提升了2m，此过程中（　　）

A．有用功是1000J

B．总功是1600J

C．滑轮组的机械效率是80%

D．拉力做功的功率是100W