启封火区时，火区内空气中的氧气浓度降到5%以下就可以进行。
参考答案：错
There is a real possibility that these animals could be frightened, ______ a sudden loud noise. [ ]
A. should there be
B. being there
C. there was
D. there having been
商港按地理位置分为（）
A、海湾港
B、转运港
C、河口港
D、内河港
E、航空港
