-He didn’t go to the concert yesterday,did he?

-______.He was busy studying for the tests all day.

A．No,he didn’t

B．Yes,he did

C．No,he did

D．Yes,he didn’t

已知A^2-离子中含有x个中子，A元素的质量数为m，则n克A^2-离子共含有电子的物质的量为（单位为摩尔）（　　）

A．

B．

C．

D．