Final answer:
You can get the flu more than once because the flu virus can mutate, resulting in new strains that evade immunity from past infections or vaccinations. This necessitates annual vaccinations to protect against the most common and new strains for each flu season.
Step-by-step explanation:
The correct statement explaining why you can get the flu more than once in your life is that The flu virus can mutate, and the flu antibodies in your system are ineffective against new virus strains. The flu virus is continuously evolving, and due to a high mutation rate in the surface molecules of the virus, each year can bring new strains that the immune system does not recognize. This explains why flu vaccines are in constant development and why it's necessary to get vaccinated annually.
Immunity developed from previous flu infections or vaccinations may not provide sufficient protection against new strains due to antigenic drift, a process by which flu viruses continuously change their surface proteins. This antigenic variation makes it challenging for the immune system to keep up, and it undermines the protective immunity acquired from past exposures to different flu virus strains.
Therefore, it is primarily the rapid mutation and evolution of the flu virus that necessitates the annual reformulation of the flu vaccine, enabling it to target the most prevalent and newly emerged flu strains each flu season.