Help me pick the word!?

Question

1. One of the problems in using solar energy is that it is difficult to (store/heat).

2. Turbines are a from of (water wheel/diesel engine).

3. Turbines are (more/less) efficient than gasoline engines.

4. All heat engines burn fuel to form a (n) (expanding/contracting) gas which pushes against a movable part.

5. (Auto/Diesel) engines use spark plugs to ignite a compressed air-gas mixture.

THANK YOU [:

Answer 1

store

water wheel

more

expanding

diesel

2 of them aren't right, i think. good luck. i hope this isn't your homework.