Based on arrow movement, why is the front shown in item A different from the front shown in item B? Warm air rises above cold air in item A, while two air masses collide in item B. Cold air mixes with warm air in item A, while two air masses barely move in item B. Warm air is pushed upward near cold air in item A, while a boundary forms in item B. Two air masses form a boundary in item A, while warm air rises above cold air in item B.