问题补充：
It was announced that only when the fire was under controlto return to their homes.
A. the residents would be permitted
B. had the residents been permitted
C. would the residents be permitted
D. the residents had been permitted
答案：
A. the residents would be permitted
B. had the residents been permitted
(答案→)C. would the residents be permitted
D. the residents had been permitted
解析：句意:这里公告只有火势被控制住了居民才可以重返家园。这里it是形式主语，that引导的是主语从句，在这个主语从句中，把only放在句首，后面的主句用部分倒装，故排除AD，根据句意居民将要返回家园，所以应该用过去将来时，故选C。
