单项选择。
He there yesterday. [ ]
A. leaves
B. left
C. leaving
答案：B
已知一个碳-12原子质量是M千克，A原子的相对原子质量是N，则A原子的质量是______g．
He has difficulty in remembering lines，but he is a good actor
A．in addition
B．all in all
C．as a result
D．at most
