根据括号里的要求改写句子。

1. The little child needn't be looked after by them. (改为主动句)

    They ________ the little child.

2. Our English teacher often tells us to speak English in class. (改为被动句)

   We ________ often ________ speak English in class by our Eng- lish teacher.

3. The room is used to play ping-pong by people. (改为主动句)

    People ________ the room ________ ping-pong.

4. We saw a lot of students playing games on the playground. (改为被动句)

    A lot of students ________ games on the playground.

5. Li Feng made the baby stop crying. (改为被动句) 

   The baby _________ crying by Li Feng.

原子吸收进行回收率测定时，加标量不得大于样品中待测含量的（）倍。

A.1

B.2

C.3

D.4