单选题 They have been waiting for many hours. The airplane must have been ______.
A. of schedule B. on schedule
C. ahead of schedule D. behind schedule

【正确答案】 D

【答案解析】本题测试介词短语的用法。根据题意，是指航班晚点，故应选behind schedule“落后于预定计划”。of schedule搭配不存在；on schedule“按时”；ahead of schedule“提前”。