下列叙述中正确的是[ ]
A.在极性分子中不可能存在非极性键
B.在离子晶体中不可能存在极性键
C.在原子晶体中可能存在离子键
D.含有共价键的晶体可能是离子晶体
答案:D
We are taught that a business letter should be written in a formal style __ in a personal one.
A.rather than
B.better than
C.more than
D.less than
我国参加国际电工委员会(IEC)是在( )年。
A.1946 B.1957 C.1993 D.1997