工程寿命周期成本分析中，为了权衡设置费与维修费之间关系，则采用的手段是()。

A．进行充分研发，降低制造费用 

B．购置备用构件，提高可修复性 

C．提高材料周转速度，降低生产成本 

D．聘请操作人员，减少维修费用

填入适当的词。

1. I haven't heard from him_______ he went to America.

2.He won't be here_______ he is invited.

3. He will not go to the cinema______ he is very busy.

4. We found the key_______ she had left it.

5. We found the books two days ______he had gone away.

6. We had no sooner got to the station ________the train left.

7. He speaks English ______ he were an Englishman.

8.He is explaining clearly______  they can understand.

9. Do not leave the room______ you have finished the test.

10. She sang______ she went along.