在通常情况下，下列物体中容易导电的是

A．玻璃棒

B．铜丝

C．空气

D．汽油

the exploration of the botanical world            on a large scale.

A．It was not until the eighteenth and nineteenth centuries that; had begun

B．It was not until the eighteenth and nineteenth centuries when; began

C．Not until the eighteenth and nineteenth centuries did; begin

D．Until the eighteenth and nineteenth centuries; began