电动机电源电压波动不得超过（）%。

A.一7～+7

B.一5～+5

C.一10～+10

D.一10～+5

Krashen identifies three kinds of affective variables related to second language acquisition:______.

A．motivation, self-confidence and anxiety

B．comprehension, acquisition and anxiety

C．input, competence and self-confidence

D．situation, motivation and self-confidence