2. 解析：选A。let out 无意间透露。故选A项。

5. It was not until World War Ⅱ that the scientists solved the problems.

提升练习

4. If penicillin had not been available, many people would have died of bacterial illness or even minor wounds.

3. I will put you through to the receptionist and she can take down your details.

2. He recommended that we (should) read books as many as possible.

4. focus on　　　 5. bring relief to

4. standardize　　　 5. arrangements

15. The young dancers looked so charming in their beautiful clothes that we took 　_______ pictures of them.

A. many of　　 　　　　B. masses of　

C. the number of　 　　D. a large amount of