问题详情

According to paragraph 4, providing a machine to take the place of the horse involved combining which two previously separate ingredients？

A．Turnpikes and canals

B．Stationary steam engines and wagons with flanged wheels

C．Metal rails in roadbeds and wagons capable of carrying heavy loads

D．Canal boats and heavily laden wagons

请帮忙给出正确答案和分析，谢谢！