盐酸小檗碱的存在形式有哪些 A．醛式 B．季铵碱式 C．酮式 D．醇式 E．烯醇式

Whenever he was asked why he was late for class，he  would answer carelessly，______ the same thing.

A．saying

B．said

C．to say

D．say

They are working hard now and I believe they will make progress                more this time.    [ ]

A. slowly    

B. quietly    

C. politely    

D. quickly

在理论上，当瓦斯浓度达到（）％，瓦斯可以和空气中的氧气完全反应，爆炸强度最大。

A、7

B、8

C、9.5

D、15

—Your sister’s plan is not quite practical.Why not try to persuade her to give it up?

—My sister is very stubborn，so it is________to try to change her mind.

A．by chance

B．in vain

C．on purpose

D．for sure

开口于下鼻道的是 ( )

A．上颌窦

B．额窦

C．鼻泪管

D．蝶窦