What to the end of the Yellow Fever epidemic of 1793?
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The government cleaned the drinking water around Philadelphia

Cold weather killed off the mosquitoes

Scientists discovered a vaccine

Eventually, a cold front eliminated Philadelphia’s mosquito population and the death toll fell to 20 per day by October 26. Today, a vaccine prevents yellow fever in much of the world, though thousands of people still die every year from the disease

The correct answer is option a.