给这栋楼供暖最经济的方式是什么？

What's the most ___economical way of heating this building?

对更省油的汽车需求日益增长。

There's increasing demand for cars that are more ___economical on fuel.

而且，自行车是一种非常经济的交通工具，因为它不需要任何燃料。

Moreover, a bike is a very ___economical means of transport because it does not need any fuel.