When the Audet family turns on the lights at Blue Farm,they

When the Audet family turns on the lights at Blue Farm,they are using electricity that comes from cows—cow manure (粪肥),to be specific.
Cows produce a lot of wastes.One cow can create 30 gallons of wastes each day.Now imagine the output of over 1,000 cows at Blue Farm.That’s really a big amount.
When farmers clean their barns,they put the wastes in a big heap,and spread some of them on their fields for fertilizer(肥料).But now places like Blue Spruce Farm have a new way of using cow wastes.They use them to produce electricity.
Here’s how it works:A big machine moves back and forth cleaning the barn floor.“The cows aren’t bothered,” says Marie Audet.“They are animals of habit; they get used to it,and just lift one foot and then another to let it go by.”
The machine pushes the manure into a big 600-gallon tank like a small swimming pool.The tank is called a digester (化粪池) because what happens there is just like what happens inside a cow:Bacteria(细菌) get to work and continue to digest the manure.
Methane gas (甲烷、沼气)in the atmosphere is known as a “greenhouse” gas because it keeps heat just like a greenhouse does,causing our planet to warm up.That’s an environmental concern.But the digester has a good result.Its gas is kept and used to power electric generators (发电机).
At Blue Farm,the generators make enough electricity to power 400 homes.The Audet family sells the extra electricity they can’t use themselves.
54.The new way of using cow wastes at Blue Farm might be best descr ibed as ______.
A.reducing B.saving C.recycling D.changing
为什么应该选择C而D不行?求详解.

C Reaycling,循环利用
D Changing,发生变化,没有循环的意思