. Tom as well as his classmates __________ a song when the teacher came into the classroom.

A．are singing

B．were singing

C．is singing

D．was singing

建设项目竣工环境保护验收时，对于评价检测结果，大气污染物综合排放标准是按监测期间污染物的( )进行评价的。

A．小时浓度
B．日均浓度
C．月均浓度
D．最高排放浓度