The weather conditions that often occur when a warm front crosses the border are () A. Continuous precipitation B. Sandstorm C. Storm D. Clear and dry.

A, because the warm air mass climbs slowly along the cold air mass, continuous precipitation often occurs when the warm front crosses the border, so it is correct;

B, sandstorm weather is a meteorological disaster that often occurs in winter and spring in northern China. Sandstorms occur for a short time and have a high intensity, which is generally caused by a cold front and does not conform to the meaning of the question;

C, the storm is mainly caused by a typhoon (cyclone), so it does not meet the meaning of the question;

D. The airflow in the anticyclone center (high pressure center) sinks, and the water vapor is not easy to condense during the sinking process, so the area controlled by anticyclone is mostly sunny, which does not meet the meaning of the question.

So choose: a