When Peter speaks in public, he always has trouble        _ the right things to say.

A．thinking of

B．to think of

C．thought of

D．think of .

鼻周期的生理意义是()

A．减低鼻腔阻力

B．增大鼻腔阻力

C．促进睡眠时反复翻身，减轻疲劳

D．增加肺泡气体交换

E．减少肺泡气体交换