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

A．should be set freeB．will be set free
C．a(chǎn)re set freeD．would be set free

C

解析試題分析：考查時間狀語從句。主句用的是一般現(xiàn)在時，從句也用相應(yīng)的現(xiàn)在時態(tài)他，排除D；before在......之前，沒來得及；引導(dǎo)時間狀語從句時，從句用一般現(xiàn)在時代替一般將來時，A項是虛擬語氣的結(jié)構(gòu)，故選C。
考點：考查時間狀語從句

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