2. Many people still believe that (1)_____________ 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 (2)_____________ find out alternative sources of power. According to Professor Marvin Burnham of the New England Institute of Technology, we (3)____________ to start conserving coal, oil, and gas before it is too late; and nuclear power is the only alternative.      However, many people don’t approve of using nuclear power (4)_____________ 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 (5)_______________ use natural resources as economically as possible. 1. A. natural B. nature C. naturally D. naturely 2. A on B. to C. for D. of 3. A. has B. had C. have D. having 4. A. so B. and C. but D. because 5. A.  should B. could C. must D. have to