John has two brothers, but either ____ out of work now. [ ]
A. are
B. is
C. has
D. have
答案:B
给下列句子选择正确的译文,并把序号填入题前的括号内。
你几岁了?[ ]
A. How old are you?
B. How are you?
有关氢原子光谱的说法正确的是( )
A.氢原子的发射光谱是连续的
B.氢原子光谱说明氢原子只发出特定频率的光
C.氢原子光谱说明氢原子能级是分立的
D.氢原子光谱的频率与氢原子能级的能量差无关