The police _____ the murderer everywhere when he suddenly appeared in a theatre. [ ]

A. is searching for

B. were searching for

C. are searching for

D. were searching

______指以前受到强化的行为现在不再受到强化，因而这种行为在将来发生的概率便降低，它是减少不良行为、消除坏习惯的有效方法。