They have produced　　　 they did last year.

A．twice more grain as

B．twice as much grain as

C．twice as many grain as

D．as twice many grain as

三相异步电动机定子各相绕组在每个磁极下应均匀分布，以达到（）的目的。

A、磁场均匀

B、磁场对称

C、增强磁场

D、减弱磁场