To keep healthy Professor Johnson cycling as a regular form of exercise after he retired.

问题补充：

To keep healthy,Professor Johnson cycling as a regular form of exercise after he retired.

A. caught on

B. set out

C. made for

D. took up

答案：

A. caught on

B. set out

C. made for

(答案→)D. took up

解析：take up意思是“占据，占领，从事”。