单项选择题 The doctor (told) him that he (should take) this medicine (one) pill at (one) time.

单项选择题 The reason (why) Jack (failed) to (attend) the lecture is (attend) he had a bad cold.

单项选择题 (It turned out) (why) the tiger (got out) of the cage because the man in charge (had forgotten) to lock it.