根据情境，选择适当句子。

1. 你向别人介绍这是你的朋友李明时，你应说：[ ]

A. He's my friend, Li Ming.

B. This is my friend, Li Ming.

C. That's my friend, Li Ming.

2. 见到李明你很高兴，你应说：[ ]

A. Nice to see you.

B. How are you?

C. How do you do?

3. 你想帮助别人时，你应说：[ ]

A. Let you help me.

B. Let me help your.

C. Let me help you.

施工、运输以及道路扬尘是扬尘污染的主要来源（）

A.对

B.错