（）是指要将输出结果的信息返回给输入，以便调整和修正系统的活动。

A.输入

B.输出

C.干扰

D.反馈

Which of the following is true of world steel production a. The steel industry still has a bright prospect on a global scene. b. Steel consumption in the world registered a drastic cut. c. Steel makers are drastically cutting down their production. d. Better promotion is needed for improving sales of steel products.