Recall Edward Jenner's Experiment, Where a Boy Was Inoculated with Cowpox

Recall Edward Jenner's experiment, where a boy was inoculated with cowpox and, as a result, did not contract smallpox when he was exposed to the virus. Why didn't the boy get smallpox?

A) because of immune "memory" provided by plasma cells
B) because of immune "memory" provided by memory cells
C) because of a primary immune response
D) because of a secondary immune response