Aplague called the black death swept medieval europe. at the time, people believed cats spread the plague (cats were associated with the devil) and started killing them to prevent the spread of disease. in reality, rats spread the plague and the slaughter of their natural predators only hastened the disease's proliferation. with this in mind, a market for cats would have had a externality because the cats

Positive

Killed regardless

Explanation:

A market for cats would have had a positive externality because the cats killed rats regardless.

Rats are considered to be found everywhere, from roadsides to kitchens in the households. Since rats in reality were the primary cause of spreading the plague known as the The Black Death, people would be positively influenced to get more cats because they are the natural predators of rats.

The more the cats, the less would be the rats and therefore less possibility of the plague getting spread.