问题补充：
It was a long time before the policewhat he was really doing.
A. caught hold of
B. caught up
C. caught on to
D. caught at
答案：
A. caught hold of
B. caught up
(答案→)C. caught on to
D. caught at
解析：句意：还要过一段时间警察才能了解他在做什么。catch hold of抓住； catch up 赶上； catch on to理解； catch at试图抓住。根据句意故选C。
