---Do you know London at all?

---No, this is the first time that I______ here．

A．was

B．came

C．have been

D．had been

把一块锌板与不带电的验电器金属杆相连后一起放在绝缘板上，当锌板被X射线照射后，则（　　 ）

A．电子由锌板逸出．

B．验电器箔片张开．

C．锌板带上负电．

D．验电器箔片带负电．