Edward Jenner Noticed That the Milkmaids Who Milked Cows by Hand

Multiple Choice

Edward Jenner noticed that the milkmaids who milked cows by hand often were infected with cowpox and that their infections were mild. He also noticed that these same milkmaids did not contract smallpox, which was a much more serious disease. He presumed that the cowpox infections prevented the milkmaids from contracting the much more serious disease of smallpox. Subsequently he began vaccinating people with the pus-like material from cowpox lesions to determine if his thinking was correct. In the story above, the underlined portion is a(n)

A) hypothesis.
B) observation.
C) theory.
D) experiment.