Physician: Stories of people developing serious health problems shortly after receiving vaccinations have given rise to the question of whether vaccination is safe. But even if these stories are true, they need not be cause for concern. With millions of people being vaccinated every year, it is to be expected that some will develop health problems purely by coincidence shortly after receiving vaccinations. Which one of the following, if true, would most strengthen the physician's argument? ['Some of the illnesses that vaccines are designed to prevent have become so rare that even if people are not vaccinated, they are unlikely to contract those illnesses.', 'The health problems that some people have developed shortly after receiving vaccinations have been more serious than the health problems that the vaccines were intended to prevent.', 'People are no more likely, on average, to develop serious health problems shortly after receiving vaccinations than shortly before receiving vaccinations.', 'For the most part, stories of people developing serious health problems shortly after receiving vaccinations involve vaccines that were recently introduced.']