Answer:
a. the polar front
Step-by-step explanation:
When the surface winds move towards the pole at angles that range from 30 to 60 degrees, they usually collide with cold air moving towards the equator. Since these two winds do not mix with ease, the polar front helps to separate them.
The three-cell model assumes that the earth is entirely filled with water thus removing any interactions with the land. It also assumes that there are no seasons nor Coriolis force.