问题详情

The law of diminishing returns is manifested by the fact that

A．the elder generations earn less than their children"s generations.

B．ordinary truck drivers have to work harder than full professors.

C．a college diploma is more promising than a high school diploma.

D．people with a B.A.earn more than those with a doctor"s degree.

请帮忙给出正确答案和分析，谢谢！