(Worth 20 points)
Which of the following explains why it is not easy to tell the directions of movement of pressure centers and fronts from a surface map alone?

A. You must know the location of major land masses such as mountains.

B. You must know the density and steepness of an approaching front.

C. You must know the variations in temperature.

D. You must know the pressure changes and the patterns of circulation of the upper air.