scientists say it may be five or six years before it is possible to test this medicine on human patients科学家对药品先要进行反复试验后才
问题描述：
scientists say it may be five or six years before it is possible to test this medicine on human patients科学家对药品先要进行反复试验后才能用于病人身上 这里的before为什么提前?为什么不直接说it is possible to test this medicine before on human patients?
答
后面的it is.是一个从句,befor而后面省略了一个that
原句是.before (that)it is.