矿尘按产生来源可分为（）两种。
A、原生矿尘
B、次生矿尘
C、可见矿尘
D、显微矿尘
E、游离矿尘
参考答案：A, B
用所给词的适用形式填空。
1. We want two good ______ (music) for our club.
2. Oh, dear! Can all the ______ (child) swim?
3. He can help kids with ______ (swim).
4. He ______ (want) to join the chess club.
5. Mary sings very well. I hope she will be a good ______ (sing).
急停按钮应安装哪些位置，选何种形式，如何安装？
