Final answer:
During the plague, miasma or 'bad air' was widely believed to be the cause of the disease. After the plague, people moved to cities due to better job opportunities that arose from labor shortages and changes in economic systems.
Step-by-step explanation:
The doctors during the time of the plague largely thought that the plague was caused by miasma (option C), which refers to bad or corrupt air. During this period, most people in Christian Europe concurred that the plague was a sign of God's displeasure, with others attributing it to supernatural forces, rare conjunctions of planets, or the poisonings by foreign or marginalized groups.
After the plague, people started moving from the countryside to cities primarily in search of better job opportunities (option A). This shift was the result of changes in the economic and feudal systems at the time. The Black Death had led to labor shortages which, in turn, gave serfs the chance to renegotiate their contracts or to settle independently, thereby laying the foundation for economic markets and capitalism.