答案：D

考查动词词组辨析。A)let up“停止，暂停”；B)let down“放松，松懈”：As his interest wore off,he began to let down. 随着兴趣的减弱，他开始松劲了。C)let off“宽恕；放（枪、炮、烟火等）”：let off fireworks放烟火；D)let out（1）“发出”：let out a cry of pain发出一声痛苦的叫声；（2）“出租”：let out cars by the day按天出租汽车。句意为：教授讲了一个笑话，班上的所有学生大笑起来。