Juan Is a 5-Year-Old Boy and Recently Recovered from the Chickenpox.He

Juan is a 5-year-old boy and recently recovered from the chickenpox.He returned to kindergarten and found out that his friend Tram had chickenpox too.Normally chickenpox is very infectious,but Juan did not acquire it again.Why was Juan protected from a second exposure to chickenpox?

A) because his immune system now has immunological memory
B) because his innate immune system was strengthened
C) because his inflammatory response was conditioned for chickenpox
D) because his immune system underwent antigenic variation