某阳离子Mn+的核外共有x个电子，核内有a个中子，则M的质量数为 [ ]
A．a+x－n
B．a+x+n
C．a－x－n
D．a－x+n
答案：B
It ______ Sunday , the Smiths will go to the seaside and enjoy themselves again .
A．is
B．being
C．had been
D．having been
中药注射剂所用原料若为从中药中提取的有效成分，其纯度应达（）
A.70％
B.75％
C.80％
D.85％
E.90％
