问题
选择题
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
C
题目分析：考查动词的时态与语态。句意：科学家们也不知道需要过多久笼子里的野生动物才能被放出来。It will be +some time +before+…+does/do 例句: It will be one and a half years before I leave school.还有一年半时间我就要毕业了。t was +some time +before 过了多久才 例句：It wasn’t long before he told me that. 没过多久他就把那件事告诉我了。再根据set free 与 wild animals之间是被动关系，所以选C项。
考点 : 考查动词的时态与语态