____ Mr. Zhang gave us some good suggestions, we still didn't deal with those problems very well. [ ]

A. Though

B. But

C. Because

D. /

呼吸时，保证气管和支气管内气流通畅的结构是( )

A．甲状软骨

B．会厌软骨

C．平滑肌

D．C形软骨环