Question

in displacement ventilation, cooler supply air is introduced at a low velocity at the floor level, displacing the warmer room air and creating a zone of fresh, cool air at the occupied level.
source: businessweek
in this sentence, velocity means:
difference between value of a property and claims against it
degree of hotness or coldness of a body or environment
elevation above sea level or above the earths surface
distance travelled per unit time in one direction

Explanation:

Velocity in a physical - context like air movement in displacement ventilation refers to the rate of motion of the air. It is the distance travelled per unit time in a particular direction. The other options are definitions of different physical or business - related concepts.

Answer:

distance travelled per unit time in one direction