The number of people invited _________ fifty, but a number of them _________ absent for different reasons.

A．were; was

B．was; was

C．was; were

D．were; were

在局部瓦斯积聚或临时停风的巷道内积聚瓦斯时，在瓦斯浓度和二氧化碳浓度（）情况下，必须采取安全措施，控制风流排放瓦斯，严禁一风吹。

A.不超过2.5%

B.不超过3

C.不超过5%