How to calculate the moving speed of the central weather system

According to the material information "The weather system moves eastward at a speed of 2 km/day in the figure" and the line scale in the figure, the width of L is about 6 km. Under the control of the low-pressure center, there may be a rainstorm. In the figure, the weather system moves eastward at a speed of 2 km/day, and it is estimated that the duration of the rainstorm in A is about 7-9 hours.

Therefore, B.