—What time will Mr. Brown be back to China?

—Sorry. I don’t know______.

A．when did he go abroad

B．why he is going abroad

C．how soon will he be back

D．how long he will stay abroad

离子选择性电极对测量阴离子时，其电位正比于（a是离子活度）（）。

A.-lnaR内

B.aR

C.-lnaR

D.lnaR