— Did you look up the time of planes to Beijing? — Yes the earliest plane is to leave at

问题补充：

— Did you look up the time of planes to Beijing?

— Yes, the earliest plane is to leave at 5:30 am.

A. likely

B. probable

C. possible

D. due

答案：

A. likely

B. probable

C. possible

(答案→)D. due

解析：本题考查形容词的用法。be due to do意为“应该…/预期的”。所以，根据句意该题应选D。