. –Tom, go to the office to fetch the papers. Who would you rather        with you, Wang Lin or Li Feng?

---        .

A．have go; Neither

B．go; Neither

C．have gone; None

D．have to go; None

如果表中一个字段不是本表的主关键字，而是另外一个表的主键字或候选关键字，这个字段称为______。