Concept:Rainfall is closely linked to atmospheric pressure conditions.
Explanation:In a low-pressure area, the air is lighter and rises upward.
As the air rises, it cools down at higher altitudes.
Cooling causes water vapor in the air to condense into tiny droplets.
These droplets gather around dust particles to form clouds.
When the droplets become heavy, they fall to the ground as rain.
Therefore, low pressure is the main condition that leads to rainfall.
Answer:A. low pressure.