The weather system measured by a weather station is () A. Cold front B.

a. The cold front is a strong cold air mass, which actively approaches the warm air mass, and its leading edge is inserted under the warm air mass, so the warm air mass is forced to rise, in which water vapor cools and condenses into clouds, causing rain. When the weather system in the figure passes through the border, the air pressure is very low, so it is controlled by low pressure, so it does not meet the question.

B. The warm front means that the warm air mass is powerful, and it actively approaches the cold air mass and rises, forming clouds and causing rain. When the weather system in the figure transits, the air pressure is very low, so it is controlled by low pressure, so it does not meet the question.

C. Cyclone has low air pressure, and the air flow direction is counterclockwise convergence and rising in the northern hemisphere (clockwise convergence and rising in the southern hemisphere), so rainy weather occurs because its central air flow is rising. When the weather system in the figure transits, the air pressure is very low, so it is controlled by low pressure, so it is correct;

D. The anticyclone has a high pressure, and the air flow direction is clockwise divergence and sinking in the northern hemisphere (counterclockwise divergence and sinking in the southern hemisphere). Because the central air flow sinks, the weather is fine. When the weather system in the figure crosses the border, the air pressure is very low, so it is controlled by low pressure, so it does not meet the meaning of the question.

Therefore, it is chosen: C.