There are two main sets of Games — the Winter and the Summer Olympics and both are held e

问题补充：

There are two main sets of Games — the Winter and the Summer Olympics, and both are held every four years on a basis.­­­­­

A. valuable

B. former

C. regular

D. swift

答案：

A. valuable

B. former

(答案→)C. regular

D. swift

解析：A．valuable有价值的，B．former以前的，C．regular定期的，有规律的，D．swift迅速的，句意：有两个主要的运动会—冬奥会和夏季奥运会，两个都是每四年举行一次。根据句意选C。