Every Year the Common Flu Vaccine Is Designed to Protect

Every year the common flu vaccine is designed to protect against three different strains of the influenza virus.Why do scientists typically have to make a new vaccine every year?

A) Our bodies develop resistance to the vaccine and it becomes less effective.
B) The virus develops resistance to the vaccine.
C) The virus can mutate into new strains.
D) People change their habits each year and become susceptible to different strains of the virus.
E) The common influenza virus can come from many species of animals including cats and dogs.