An individual receives a flu vaccine and is well protected for the current flu season. The following flu season, the individual fails to get another flu vaccine and comes down with the flu. What is the most likely explanation for vaccine ineffectiveness in the subsequent flu season?

A) Antibiotic-resistant bacteria were able to overcome the effects of the vaccine.
B) The immune system becomes weaker when continually exposed to the same virus contained in the vaccine.
C) The flu virus mutated, leading to a new strain of virus that could escape detection by the immune system.
D) Most vaccines have a limited lifetime and eventually break down and become ineffective.