Answer:
two-stage area sampling
Step-by-step explanation:
A two-stage area sampling is a type of sampling method that is employed when dealing with a large population that is being studied. In this type of sampling, two stages of the sampling process is involved. The first stage involves dividing the population under study into clusters, and then randomly selecting few clusters you want to focus on. The second stage involves, random selection of your subjects in each of the clusters that you have randomly selected out.
This is done to ensure, as much as possible, the population under study is adequately represented, especially when dealing with a large population size such a country, state, or region.
The type of sampling described in the question above is a two-stage area sampling.