问题补充：
—What did he say just now? —He said .
A. what time the library was build.
B. he has made many mistakes in the exam.
C. if the teacher would ask him to go to the office the next day
D. his mother had waited for him for 20 minutes at the school gate.
答案：
A. what time the library was build.
B. he has made many mistakes in the exam.
C. if the teacher would ask him to go to the office the next day
(答案→)D. his mother had waited for him for 20 minutes at the school gate.
解析：句意：----刚才他说什么了？-----他说他的妈妈在学校门口等了他20分钟了。从主句的谓语动词said看出没有任何疑问，所以排除选项A,C；当主句用的是一般过去时态时，宾语从句要用过去的某种时态，所以B不对。故选D。
