折射角θ2（）90°的入射角叫做临界角。A、大于 B、等于 C、小于 D、以上都对

Johnson had made up his mind to give it up，but on______second thought he determined to try______ third time.

A．/；a

B．/；/

C．a；a

D．the；a

室内空气污染物中与肺癌发病密切相关的是( )

A．SO₂
B．²²²Rn
C．CO
D．NO_x
E．甲醛

“Wechat”(微信)really makes a great _____to people’s life. It’s easier for us to keep in touch with others.

A．choice

B．difference

C．problem

D．surprise

当锅炉灭后，炉膛负压会突然（）

A.减小

B.减小后增大

C.增大

D.剧烈波动

建筑工程绿色施工评价要素包括下列哪些指标（）。

A、控制项

B、一般项

C、合格项

D、优选项