线圈内感应电动势的方向可由（）定律确定。
A、基尔霍夫电流
B、电磁感应
C、基尔霍夫电压
D、楞次
参考答案：D
The people have become masters of their country. _____ science can really serve the people. [ ]
A. It is only then; that
B. It was that; when
C. It is only when; that
D. It was when; then
中性甲醛液不含下列哪种物质？（）
A.磷酸氢二钠
B.40％甲醛
C.磷酸二氢钠
D.无水乙醇
E.蒸馏水
