情景选择。

1. 别人过生日时，你应对他说：                                [ ]

A. How are you?

B. This is for you. 

C. Happy birthday to you!

2. 你想知道别人多大时，你应该问：                              [ ]

A. How many?

B. How old are you?

C. How are you?

3. 送给别人东西时，你说：                            [ ]

A. This is for you!

B. How many gifts?

C. Thank you!

4. 当别人谢你时，你说：                            [ ]

A. Nice to meet you.

B. You are welcome.

C. Great!

带传动的打滑是如何发生的？它与弹性滑动有何区别？打滑对带传动会产生什么影响？