A body which falls from a height has a constant acceleration
The part of the moon which faces the sun may reach the boiling temperature of water.
The speed at which sound travels varies with the direction of the wind.
(Which) stands for things and can be preceded by a preposition.
Helium is the safest gas that is used in balloons.
The architect that designed the stadium was Mr. Reed.
It is the ionosphere that reflects radio waves back to the earth.
(That) stands for persons and things. It must be used when the noun it refers to is preceded by an adjective
in the superlative degree of by (same), (first) or (last) .
It is also used after the expression :