问题详情

How did Dr. Edward Jenner solve the problem of smallpox？
A. He kept on focusing on people who had smallpox.
B. He changed his way of thinking by turning to people without smallpox.
C. Dairymaids advised that he use cowpox to experiment.
D. He happened to discover cowpox and he experimented with it on dairymaids.请帮忙给出正确答案和分析，谢谢！