Scientists also have no idea how long it will be before all wild animals in the cages . A.

问题补充：

Scientists also have no idea how long it will be before all wild animals in the cages .

A. should be set free

B. will be set free

C. are set free

D. would be set free

答案：

A. should be set free

B. will be set free

(答案→)C. are set free

D. would be set free

解析：考查时间状语从句。主句用的是一般现在时，从句也用相应的现在时态他，排除D；before在......之前，没来得及；引导时间状语从句时，从句用一般现在时代替一般将来时，A项是虚拟语气的结构，故选C。