b. There is insufficient contrast in surface type (land vs. ocean) in the middle latitudes.
Explanation:
The sources of air masses can be produced or at very high latitudes because they can carry big amounts of air from the top of a mountain till the bottom by the differences of pressure and temperatures. This kind of air mass is called continental.
In the other hand, in a low latitude, the air coming from the sea, try to reach higher latitudes, again, by pressure and temperatures differences. These kind of air masses are called maritime.
