Luggage may not be brought into the building _______

试题详情及答案解析

Luggage may not be brought into the building ________ a guard has checked it first.

A．when

B．because

C．after

D．unless

答案：D

试题分析：考查状语从句。A. when 当---时候；B. because 因为；C. after在---之后；D. unless除非。句意：行李不可能被带进大楼，除非一个哨兵首先检查了它。故选D。
考点：考查状语从句。