Answer:
In item A, two air masses form a boundary; whereas, in item B, warm air rises above cold air.
Step-by-step explanation:
When warm and cold air masses meet in the opposite direction, they form a boundary but warm air would still rise above the cold air, and cold air would stay below the warm air.
On the other hand, when both air masses move in the same direction, warm air would stay above the cold air and they both will move forward.