国家标准规定普通硅酸盐水泥烧失量控制（）.
A.≤4.5%
B.≤5.0%
C.≤4.0%
D.≤3.5%
参考答案：B
His trousers _______ worn out.[ ]
A. is
B. are
C. be
D. was
电压降落的纵分量主要受哪个因素影响？（）
A.无功
B.有功
C.频率
D.电流
