D. It cools down and descends.
High-pressure systems occur when air cools down and becomes denser, causing it to sink toward the Earth's surface. As the air sinks, it becomes compressed and warms up due to the increase in pressure. This warming process can cause the humidity in the air to decrease, leading to clear skies and dry conditions.
As the air continues to sink and spread out, it creates a region of high pressure at the surface. This high-pressure system typically leads to stable weather conditions, as the sinking air inhibits the formation of clouds and precipitation. In contrast, low-pressure systems are associated with rising warm air, which can lead to the formation of clouds and precipitation.