The main idea of this passage is ______.

A．Wood-bunting stoves are safer and more efficient than they used to be.
B．No factories exist for the purpose of producing dirty wood chips.
C．Both catalytic combustors and wood agitators save money in the long run.
D．Wood-burning stoves with catalytic combustors cost between $ 100 and $ 200.

A．many Americans have switched from hydrocarbon-based fuels to wood because the price of the latter has risen
B．some wood-burning stoves cost up to one hundred times more than others
C．catalytic combustors increase the amount of pollution caused by wood burning stoves
D．agitators are cheaper addition to wood-burning stoves than catalytic combustors