按要求完成下面各句。每空一词。

1. Martin used to give his mother a lot of trouble. (改为否定句)

    Martin _____ _____ to give his mother a lot of trouble.

2. To keep healthy, more and more people prefer walking to driving. (同义句转换)

    To keep healthy, more and more people would walk _____ drive.

3. She learns English by listening to tapes. (对划线部分提问)

    _____ she learn English ?

4. His parents didn't catch his words at once. (改为被动语态)

    His words _____ _____ by his parents at once.

5. Tom asked me n:ot to stand here. (把间接引语改为直接引语)

    "_____ stand here." Tom _____ to me.

简述SDH传送网中电路层的作用。