( )由上槛、下槛、立筋(也称龙骨或墙筋)、斜撑或横档组成骨架，然后将板条钉在骨架上，最后在板条两侧抹灰而成。

A．灰板条隔墙

B．砌块隔墙

C．加气混凝土隔墙板

D．普通砖隔墙

__________ you understand the rule, you will have no further difficulty.

A．While

B．Once

C．Though

D．Unless