31．—Can you tell me what time the flight to Detroit leaves?

—I'm sorry, but it ______．

A．has been canceled

B．had been canceled

C．has canceled

D．had canceled

厚角组织的增厚部分的化学成分是（）。

A.纤维素

B.半纤维素

C.果胶质

D.果胶质和纤维素