下列分子的偶极矩最小的是( )。
A．SO2 B．B2H6 C．NCl3D．SF4
参考答案：B
解析： 分子完全对称，偶极矩为0。
He doesn't read the English text very often. ______ he has failed again. [ ]
A. It is why
B. That's because
C. It is reason
D. That's why
不能有效清除菌斑的机械方法是
A．清水含漱
B．牙刷
C．牙膏
D．牙线
E．牙签
