单相桥式可控整流电路电阻性负载，晶闸管中的电流平均值是负载的（）倍

A．A．90°

B．B．60°

C．C．120°

D．D．300°

A.1/6

B.1/4

C.1/2

D.1/3

A.1

B.1/2

C.1/3

D.1/6

A.1/3

B.1/2

C.1/6

D.1/4

A.90°

B.120°

C.150°

D.180°

A．A．180°-α

B．B．90°-α

C．C．90°+α

D．D．180°+α