After a Visit with Her Niece Who Had an Active

After a visit with her niece who had an active upper respiratory infection, Emma was worried that she would become ill as well. After a week, she still had no symptoms. What might explain Emma's response to exposure to this respiratory virus?

A) This was her first exposure, and her immune system underwent a primary immune response.
B) She had been exposed to this virus before. Her immune system underwent a secondary immune response.
C) She had been exposed to this virus before. Her immune system underwent a hypersensitivity response.