He could have finished it on , but he was ill last week and that was why he fell behind.
A. schedule
B. disadvantage
C. purpose
D. timetable
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(答案→)A. schedule
B. disadvantage
C. purpose
D. timetable
解析：词义辨析。A. 时间表 B.缺点 C.目的 D. 时间表；课程表。句意：他本能按时完成，但是上周他生病了，那是他为什么落后了。短语on schedule，按时，故选A。