唾液中的主要无机成分是（）。
A.钠、钾、氟
B.钠、钾、氯
C.钙和磷
D.氟
E.重碳酸盐
参考答案：B
The more ______ we take the safety standards, the further we can speed up the development of the school bus industry.
A．constantly
B．generally
C．seriously
D．immediately
A．甲光的频率比乙光的频率大
B．相同条件下，甲光比乙光容易发生衍射
C．对同种介质，甲光的折射率比乙光的折射率大
D．在同种介质中甲光光速比乙光光速小
