Answer: The correct answer is D. He created a vaccine against polio.
Explanation: In 1954, Salk undertook a large-scale national study, enrolling over one million pediatric subjects. The next year, on 12 April 1955, he announced the results: the vaccine was both safe and efficacious. Subsequent data showed that in 1955, there were approximately 29,000 cases of poliomyelitis in the US.