Jon is working on a city councilperson's election campaign. He wants to survey potential voters about their opinions. Which is the best way for him to gather a sample?
A. attend a speech given by the councilman and survey audience members
B. select every 10th person on a list of registered voters and call them to complete the survey
C. go to a gym during the day and survey people as they exercise
D. hand out flyers at the supermarket asking people to call and complete a survey