The symptoms of hepatitis A appear no earlier than 60 days after a person has been infected. In a test of a hepatitis A vaccine, 50 people received the vaccine and 50 people received a harmless placebo. Although some people from each group eventually exhibited symptoms of hepatitis A, the vaccine as used in the test is completely effective in preventing infection with the hepatitis A virus. Which one of the following, if true, most helps resolve the apparent discrepancy in the information above?
A: The people who received the placebo were in better overall physical condition than were the people who received the vaccine.
B: Of the people who developed symptoms of hepatitis A, those who received the vaccine recovered more quickly, on average, than those who did not.
C: The vaccinated people who exhibited symptoms of hepatitis A were infected with the hepatitis A virus before being vaccinated.
D: The placebo did not produce any side effects that resembled any of the symptoms of hepatitis A.
Choose A, B, C or D as your solution.
C