Which sampling method is most appropriate in estimating the average number of votes for each candidate in an election across a county of three towns if town a has one million retirees, town b has two million business owners, and town c has three million office workers?

simple random sampling, because the people in the sample are accessible and available

stratified sampling, because there are specific subgroups to investigate

systematic sampling, because it is difficult to identify items using a simple random sampling method

cluster sampling, because the studied population is spread across a wide area such that simple random sampling would be difficult to implement

all the above sampling methods