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.

in the year 2850, humans successfully colonized mars. the martian modules that were constructed could hold only a small population of people. it is now a century later, and the population on mars has grown to 600 people. of the current mars population, 45 people express the recessive phenotype for phenylketon- uria (pku) syndrome (pp genotype).

explanation: