单项选择题

Questions 49 to 52 are based on the following passage: Radiant energy is also called light energy. Our most important source of radiant energy is, of course, the sun. Without it there would be no life, for plant life depends upon radiant energy--and we depend upon plant life. Other forms of radiant energy familiar to us are radio waves, infrared rays, ultra-violet rays, X rays, and gamma rays. In simplest terms, mechanical energy is the energy of a moving object. Your heartbeat, the pushing of a lawn mower, a baseball speeding through the air, water falling onto a waterwheel, the prying up of a rock with a crowbar--all are mechanical energy, which, of course, was another form of energy earlier. Electrical energy usually has to be changed to another form before it actually does what we want it to do. In the toaster, it changes to heat; in a tiny wrist watch or a subway train or a 75,000 horse-power industrial motor, it changes to mechanical energy. In an industrial process called electrolysis, it changes to chemical energy. Various fuels such as coal, gas, oil, and food are common examples of chemical energy. These fuels give off heat when they burn. The process is called combustion or oxidation, and the heat itself is a form of energy. Man uses the energy from some kind of fuel for everything from flying to the moon to thinking about it. A special kind of chemical energy is nuclear energy, sometimes called atomic energy. These names come from the fact that the source of energy is the nucleus, or core, of the atom. Nuclear fuels such as plutonium and uranium give off heat energy when fission (the splitting of atoms in these fuels) occurs. Already we have seen tremendous advances in the use of nuclear energy for peaceful purposes, and many people see nuclear energy as the future answer to almost all of our energy needs. From the facts given in this article we can tell that __