Downward flow is very popular in crisp autumn.

This question examines common weather systems. In the vast areas of northern China, it is sunny in autumn and sunny in the downflow. Cyclone and anticyclone describe the airflow condition of weather system, and low pressure and high pressure describe the pressure condition of weather system. In fact, cyclone is a low-pressure system and anticyclone is a high-pressure system. High pressure system is dominant in downstream. So choose option a in this question.