Based on arrow movement why is the front shown in item a different from the front shown in item b?

In item A, two air masses form a boundary; whereas, in item B, warm air rises above cold air.

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.

they had the law tat factories that produces smoke cannot be built near residential buildings.

building of proper sewage system was done to ensure that the waste is carried away from the humans so that they do not get diseases from it.

also they built water systems to provide water in those areas which are drought prone.