The city council for a large city wanted to know what proportion of the residents thought fitness classes should be available to everyone. The research department offered to cooperate in conducting a survey, and a simple random sample of 500 voters was selected from all the registered voters in the city. A survey form was sent by mail to these 500 voters. In this case, the population of interest is: A. the residents who responded to the mail survey B. all the residents who think that fitness classes should be available to everyone. C. all the residents of the city. D. all the residents on the list of registered voters. E. the 500 residents who got the mail survey.