Doubting that the traveler carried drugs, the Customs officer stopped him and _______ his suitcase, but found nothing at all.

A．looked up

B．went through

C．searched for

D．referred to

（）将有机体的行为分为两类：应答性行为和操作性行为。操作性条件反射建立的规律是：如果一个操作发生后，接着给予一个强化刺激，那么其强度就增加。