________ anything like this style of architecture before, we thought we were looking at giant sculptures, not buildings.

A．We had never seen

B．Never having seen

C．Never being seen

D．Never had we seen

如何用化学方法鉴别巴比妥，苯巴比妥，司可巴比妥，异戊巴比妥和含硫巴比妥？