Which sentence from the passage best supports the idea that before Salk's team, medical researchers had been unable to combat polio effectively? A. Building on the research of previous scientists and doctors, Salk developed a polio vaccine by growing large numbers of the virus in test tubes.

B. Analyzing their test results, Salk and his team were thrilled to see that their vaccine caused the immune system to produce polio antibodies without making a person ill.

C. In 1952, the United States dealt with 57,000 cases of polio—the most cases ever in a single year.

D. The next year, scientists announced that the vaccination trial had been a success: the vaccine prevented polio infections in 80% to 90% of people.