“轻水”泡沫由（）组成。
A.氟碳表面活性剂
B.碳氢表面活性剂
C.水
D.溶剂
参考答案：A, B, C, D
There are more than three thousand students in my school, most of____ from the country.
A．that
B．which
C．whom
D．them
开展教诲工作，首先需要（）。
