A、Coal burnt daily for the comfort of our homes. B、Anything that can be used to produce power. C、Fuel refined from oil extracted

W: What does the term "alternative energy source" mean?
M: (13) When we think of energy or fuel for our homes and cars, we think of petroleum, a fossil fuel processed from oil removed from the ground, of which there is a limited supply. But alternative fuels can be many things. Wind, sun and water can all be used to create fuel.
W: Is the threat of running out of petroleum real?
M: It has taken thousands of years to create the natural stores of petroleum we have now. We are using what is available at a much faster rate than it is being produced over time. The real controversy surrounding the amount of petroleum we have is how much we need to keep in reserve for future use. (14) Most experts agree that by around 2025, the amount of petroleum we use will reach a peak. Then production and availability will begin to seriously decline. This is not to say there will be no petroleum at this point. But it’ll become very difficult and therefore expensive to extract.
W: Is that the most important reason to develop alternative fuel and energy sources?
M: There are two very clear reasons to do so. (15) One is that whether we have 60 or 600 years of fossil fuels left, we have to find other fuel sources eventually. So the sooner we start, the better off we will be. The other big argument is that when you burn fossil fuels, you release substances trapped in the ground for a long time, which leads to some long-term negative effects, like global warming and the greenhouse effect.
13. What do we usually refer to when we talk about energy according to the man?
14. What do most experts agree on according to the man?
15. What does the man think we should do now?