给这栋建筑供暖最经济的方式是什么？

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

对更省油的汽车的需求正在增长。

There is growing demand for cars that are more ___economical on fuel.

自行车是一个经济的选择，因为它不需要燃料。

A bicycle is an ___economical choice as it requires no fuel.