I must have been a great_________to my parents in those hard days, for I never failed to live up to their expectations.

A．achievement

B．comfort

C．disappointment

D．support

下列关于隧道的说法正确的是（）。

A.隧道洞内空气中瓦斯浓度达到爆炸限度就会爆炸

B.隧道施工时遇到暗河或溶洞有水流时，宜排不宜堵

C.有流沙突泥地带隧道施工时可直接开挖施工

D.膨胀土吸水能力差