[单项选择]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 calle
A. often a form of energy is useless to us until it becomes another form
B. steam power is cheaper and better than electrical power
C. in the near future, there will be enough nuclear power to supply all our energy needs
D. energy can change to another form, but the new form cannot change hack into the old form