39. Thank you for your help. ______ for your help, I wouldn’t have succeeded in my experiment.

A．If it is not

B．Were it not

C．Had it not been

D．If they were not

滑翔机与飞机的根本区别是，它升高以后（）靠自身重力在飞行方向的分力向前滑翔。虽然有些滑翔机装有小型发动机（称为动力滑翔机），但主要是在滑翔飞行前用来获得初始高度。

A.用动力

B.不用动力

C.可用动力也可不用动力