Living in the mountain area has its problems, ________ obtaining water is a big problem.

A．of which

B．for what

C．as

D．whose

当焊接热输入过大时，过热区的晶粒尺寸和韧性的特点是（）

A.晶粒粗大，韧性降低

B.晶粒细小，韧性提高

C.晶粒粗大，韧性不变

D.晶粒不变，韧性不变