The young man stopped and looked back as if ______ whether he was being followed.

A．seeing

B．to have seen

C．having seen

D．to see

慢性支气管炎可出现（）

A．支气管“轨道征”

B．空气支气管征

C．印戒征

D．原发复合征

E．支气管截断征

F．空气半月征

G．垂柳征