--- What’s the matter, Mary? You looked upset.

--- I failed again in my math exam. I’m really sorry because I don’t want to ____ my parents down.

A．turn

B．write

C．let

D．take

处于病程发展较早阶段的慢性阻塞性肺病通常是（）。

A.FEV1值下降

B.气道受损严重

C.有可靠的体征

D.无症状的