Answer;
A. density.
High- and low-pressure areas leading to air movement happen when there are differences in density.
Explanation;
- Atmospheric pressure or air pressure is the force exerted by air above a surface as gravity pulls it to Earth.
- Air pressure decreases with an increase in altitude. Atmospheric pressure is measured using a barometer and is an important indicator of weather.
- When a low-pressure system moves into an area, it causes cloudiness, wind, and precipitation. High pressure systems on the other hand causes a fair, calm weather.