Which of the following metaphors could the author most appropriately use to summarize his

Which of the following metaphors could the author most appropriately use to summarize his own assessment of the American economic system(lines 42-72)？

A．A windmill.

B．A waterfall.

C．A treadmill.

D．A gyroscope.

E．A bellows.

请帮忙给出正确答案和分析，谢谢！