普通混凝土的抗拉强度为抗压强度的（）。
A.1/5~1/10
B.1/10~1/20
C.1/20~1/30
D.1/30~1/40
参考答案：B
—______ does Tim go to a movie?
—Twice a month. [ ]
A. How long
B. How soon
C. How often
研究计划的内容。
