问题补充：
Tom didn go to school this morning because he overslept.
A. at once
B. on time
C. by accident
D. at present
答案：
A. at once
(答案→)B. on time
C. by accident
D. at present
解析：考查词语辨析。at once “立刻；马上”；on time“按时”；by accident“偶然”；at present“目前;现在”。句意为“汤姆今天早晨没有按时上学，因为他睡过了头”故选B.
