In a large city school system with 20 primary schools, the school board is considering the adoption of a new policy that would require students to pass a test in order to be promoted to the next grade. The parent teacher association (PTA) wants to find out whether parents agree with this plan. The PTA randomly selected 20 parents from each elementary school. Sent them a survey, and followed up with a phone call if they did not return the survey within a week. Which sampling method was used? [Choose] [Choose) Simple random sampling Previous Stratified sampling Next Systematic sampling