AuthenticD3mon and I talked it over and agreed that
natural gas is the least polluting fossil fuel.
(See comments under his answer.)
Answer:
The relationship between acceleration and time relates to the velocity and how it changes throughout the movement of an object.
Two major characteristics that change when air is heated and cooled is the density and the direction of the air flow. They change the movement of the air by heated air rises and cooled air doesn't.
Displacement is the change in position