Answer:
Multistage sampling D
Step-by-step explanation:
The researcher used two-stage cluster sampling method; the researcher first selected randomly 20 counties from all the counties, then the researcher selected a subgroup of 3 high schools from each selected county and sample all the students in it. If it single-stage cluster sampling, the researcher randomly select a group from the large group and sample all the element.
There is also another type of cluster sampling called multi-stage sampling. This a sample method in which the researcher first divide the population into districts, from the district, a group is randomly selected, from the smaller group is again selected.