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%