In the following four schematic diagrams of weather system, what caused the unique "summer drought" weather in the middle and lower reaches of the Yangtze River?

Analysis of test questions: the summer drought affecting the Yangtze River basin in China is due to the influence of subtropical high, and the downward airflow with high temperature and little rain prevails; Fig. a shows that the updraft is dominant in the vertical direction and is judged as the low pressure center; In Figure B, the horizontal airflow converges and is judged as the low pressure center; C stands for front; The horizontal airflow in Figure D is divergent and the wind direction is to the right, so it is judged as the high pressure center map in the northern hemisphere.

Comments: This question is generally difficult. Students only need to master the common celestial system types that affect the formation of special weather in some areas of China, and judge the weather system by combining the characteristics of airflow movement illustrated in the options. Pay attention to the relationship between air pressure distribution and horizontal and vertical airflow direction.