Mrs. Smith locked both the front and the back doors before she went to bed ________ that a thief might steal into the house.

A.so

B.for fear

C.unless

D.in case

答案及解析： B

解析：

for fear that“准恐”引导一个表示动作原因的状语从句，相当于because sb. fears that或because sb. is afraid that…