阅读短文，从A、B、C、D四个选项中选择最佳答案。

    Mr. White and his 1 wanted to do it by decided to paint the outside of their house. To save money they 2. On Saturday morning they bought some paint and two brushes. They began that afternoon 3 the back of the house. The next Saturday 4 went to a football match while his wife painted the 5 of the house. On Sunday they found they couldn't open 6 of the front windows. They got them all 7 in the end. But they 8 three of the seven which were very expensive to repair. Next time 9 10 try to save money, they'll certainly pay to do the work.

关于功率和机械效率，下列说法正确的是（　　）