Olympic Winter Games _______ in the world every four years.

选择一项：
a. are held 解析：句意：冬奥会每四年在世界上举办一次。考查被动语态。主语“Olympic Winter Games”与动词“hold”存在被动关系，且时间状语是“every four years”可知，此处要用一般现在时的被动语态，即be+过去分词；主语“Olympic Winter Games”表示复数，be动词用are，hold的过去分词为held。。
b. were held
c. holds
d. held