The graph shows the antibody levels in the blood after a person has been exposed twice to the same pathogen. With regard to this pathogen we can conclude that

A. the person has a deficient immune response.

B. the virulence of the pathogen increased between the first and second exposures.

C. antibodies are only produced after a second exposure to the pathogen.

D. memory cells for antibodies against the pathogen exist at the time of the second exposure.