一束光从真空射人某介质，当入射角为45°时，其折射角为30°，则该介质的折射率为

I thought the teacher would say something about our exam result but he ________ it.

A．doesn't mention

B．hasn't mentioned

C．wouldn't mention

D．didn't mention

已知z₁=m²－3m+m²i,z₂=4+(5m+6)i,其中m为实数,i为虚数单位,若z₁－z₂=0,则m的值为

A．4

B．－1

C．6

D．0

剪切后角有和作用？

活塞式压缩机排气阀漏气，会引起（）

A.吸气温度升高

B.吸气压力降低

C.排气压力升高

D.功率消耗大

使证券的发行交易发挥着调节资本流向作用的是其 ( )

A．权益性

B．流通性

C．机会性

D．风险性