[求助] CFA L1 Ecomonics 问题请教

Schweser June 08 Exam 2 Afternoon

39. The law of diminishing marginal returns explains:

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

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

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

D. the upward sloping portion of the long-run marginal cost curve.

B is the answer.  Why?