Which of the following infinitives（不定式)expresses REASON？A．He searched the room only to

Which of the following infinitives(不定式)expresses REASON？

A．He searched the room only to find nothing.

B．I come here only to say good-bye to you.

C．I"m glad to see you.

D．He ran so fast as to catch the first bus.

请帮忙给出正确答案和分析，谢谢！