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

A. The cold front is a powerful 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, and the water vapor cools and condenses into clouds in it, causing rainfall. When the weather system passes through the boundary, the air pressure is very low, so it is controlled by low pressure, which does not meet the meaning of the question;

B the warm front is a powerful warm air mass, which actively approaches the cold air mass and rises to form clouds, causing rainfall. When the weather system passes through the boundary, the air pressure is very low, so it is controlled by low pressure, which does not meet the meaning of the question;

C. Cyclone is low pressure, and the airflow direction in the northern hemisphere is counterclockwise convergence and upward (clockwise convergence and upward in the southern hemisphere), so rainy weather occurs because its central airflow is rising. The weather system in the picture is controlled by low pressure when crossing the border, so it is correct;

D the anticyclone has a high air pressure, and the airflow direction diverges clockwise in the northern hemisphere (diverges counterclockwise in the southern hemisphere). It's sunny because the central air current sinks. When the weather system in the picture crosses the boundary, the air pressure is very low, so it is controlled by low pressure, which does not meet the meaning of the question.

So choose: C.