—Excuse me, how can I get to the museum?

-- Sorry. It's my______ time to he here.

A. one               B. first               C. once

损伤性血气胸呼吸困难的主要原因是

A．气体交换容量减少
B．回心血量减少
C．心脏排血阻力增加
D．肺表面活性物质减少
E．肺脏弥散功能障碍