Compared to people who live in other environments, people who live in urban areas have high rates of infection. Such people also have among the lowest levels of physical fitness. Clearly, infection can result from low levels of physical fitness.

Which of the following, if true, most seriously weakens the argument above?


Rates of infection vary widely among different urban areas.


People who live in urban areas are exposed to extreme concentrations of pollutants, which are known to cause infection.


Many people who live in urban areas have high levels of physical fitness.


Some of the cases of infection in people who are very physically fit are not documented.


Infections are the most infrequently occurring form of illness in either rural or urban areas.