承插式钢管脚手架
参考答案：
采用承插连接的钢管脚手架。
有时引起极化的原因可能只有浓差极化。
There _____ a number of people in the bath room. The number of them _____ almost fifty. [ ]
A. are; is
B. is; is
C. is; are
D. are; are
