The law of diminishing marginal returns explains:（）

A. the shape of the long-run average cost curves.

B. the upward sloping portion of the short-run marginal cost curve.

C. the initial decrease in the short-run average costs.

常用的钢材制品有（）、钢管、螺旋连结件及其它紧固件等。

A、各类建筑钢筋

B、钢丝

C、钢绞线

D、型钢