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) because our bodies develop resistance to the vaccine and it becomes less effective
B) because the virus develops resistance to the vaccine
C) because the virus cannot mutate into new strains
D) because people change their habits each year and become susceptible to different strains of the virus
E) because the common influenza virus can come from many species of animals,including cats and dogs