Correct Option: A

Jonas Edward Salk was an American medical researcher and virologist, best known for his discovery and development of the first polio vaccine. The field trial set up to test the vaccine developed by Salk and his research team was the most elaborate program of its kind in history, involving 20,000 physicians and public health officers, 64,000 school personnel, and 220,000 volunteers,” with over 1,800,000 school children participating in the trial. On April 12, 1955, Dr. Thomas Francis, Jr., of the University of Michigan, the monitor of the test results, “declared the vaccine to be safe and effective.”