________ stop advertising, prices would be greatly reduced.

A．Were they to

B．Could they

C．If they

D．Would they

列车运行中，轴承的内外圈与滚动体之间形成滚动摩擦，不可避免地产生（）。

A.摩擦热

B.运动热

C.自然热

D.正常热