To measure physical activity behaviors of primary school teachers in Victoria, researchers acquired a full listing of all primary schools in Victoria. The team then randomly selected 100 of these schools. Within each school, individuals were divided into groups based on age and a further random sample of 20 individuals within each of the groups was selected to enrol in the cross-sectional study. What type of sampling method/s was/were used here? Question 9 options: a) Stratified sampling then cluster sampling. b) Cluster sampling then stratified sampling. c) Simple random sampling. d) Stratified sampling then simple random sampling