It is difficult for the students______  the homework on weekends.

A．finish

B．to finish

C．finishing

D．finished

阻塞性肺气肿产生低氧血症的主要机制是（）

A.膈肌运动幅度降低

B.通气／血流比例失调

C.肺活量减少

D.肺总量减少

E.胸部活动受限