Which of the Following Contributed to Several Measles Epidemics in the 1990s

Which of the following contributed to several measles epidemics in the 1990s?

A) People had stopped getting immunizations for measles and other diseases.
B) The measles vaccine was no longer effective against the agent because the virus had mutated.
C) The measles vaccine did not provide lifetime immunity.
D) Thousands of ineffective contaminated vaccine doses had unknowingly been injected.