Air of the same mass passes through the upper surface of the outlet at the same time. Because the air outlet B is straight and the air outlet A protrudes upward, the air flow through the upper surface of the air outlet B is small and the pressure is high. After passing through the upper surface of the outlet A, the speed is high and the pressure is low, and a pressure difference to the right is formed between the two outlets. Therefore, the wind on the ground always blows from port B to port A. 。
So choose B.