沉淀池和澄清池的个数或能够单独排空的分格数不宜少于（）。
A.同时工作的个数
B.三个
C.两个
D.四个
参考答案：C
David comes from Australia. He is an _____________.[ ]
A. Englishman
B. American
C. Australian
Smoking is one of the________causes of cancer, killing millions of people each year. [ ]
A. major
B. similar
C. commercial
D. chemical
