—Excuse me, Mr Li, I don't know the meaning of the whole sentence. What should I do?

—You'd better first ____the new word. [ ]

A.look for

B.look up

C.look through

D.look at

关于磁场的产生，下列说法中正确的是（   ）

A．电场一定能产生磁场

B．电荷一定能产生磁场

C．运动的电荷可以产生磁场

D．周期性变化的电场可以产生磁场