Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the following questions

Many people still believe that natural resources will never be used up. Actually, the world's energy resources are limited. Nobody knows exactly how much fuel is left. However, we also should use them economically and try to find out alternative sources of power. According to Professor Marvin Burnham of the New England Institute of Technology, we have to start conserving coal, oil and gas before it is too late; and nuclear power is the only alternative.

However, many people do not approve of using nuclear power because it is very dangerous. What would happen if there were a serious nuclear accident? Radioactivity causes cancer and may badly affect the future generations. The most effective thing is that we should use natural resources as economically as possible.

How much fuel is left?