課后練習：

1. If ____ to define it, we’dprobably respond with something like…

A asked B asking C ask D be asked

2. One of the most fascinatingthings about _____ ability to change from one form to another.

A is its B it its is C it is its D its it is

3. This principle of change iswhat keeps everything ______in this world.

A to go B to going C going D to going

4. When you pop a ___ waffle intothe toaster, electrical energy turns into heat energy.

A frozen B freeze C freezing D frugal

5. Thus, energy readily changes_____ from one form to another.

A back and forth B back to forth

Cback to back D black and forth

6. Without light energy there_____ no life.

A would be B is C are D will be

7. Your heartbeat, a baseball____ through the air, the water ___ onto a waterwheel, the ____ up of a rockwith a crowbar--all are mechanical energy, ____, of course, was another form ofenergyearlier.

A speeding, falling, prying,which B speed, fall, pry, that C speeding, falling, prying, that

8. Electrical energy usually hasto be changes to another form before it actually does ______.

Awhat we want it B what we want it to do

Cwhich we want D which we want it to do

9. These fuels ____ heat whenthey burn.

Agive up B give in C give off D give out

10. Man uses the energy foreverything ____ flying to the moon to ____ about it.

A from, thinking B from, think C that, think D that, thinking

11. These names come from thefact _____ the source of energy is the nucleus, or core, of the atom.

Awhich B that C what D what

12. Energy—the kind of magic wecan’t live ____.

A without B with it C within D without it

答案：1 A 2 C 3 C 4 A 5 A 6 A 7 A 8 B 9 C 10 A 11 B 12 A