单肺通气时气道压力应最高不超过（）
A.10cmH20
B.20cmH20
C.30cmH20
D.40cmH20
E.50cmH20
参考答案：D
The book, ___________ is green, has been lent to my brother.
A．of which cover
B．the cover of which
C．whose covers
D．its cover
