In order to obtain a sample of voters in Pennsylvania, a simple random sample of 10 counties is selected. From each of the selected counties, 10 precincts are chosen at random. Finally, from each of these 100 precincts , a simple random sample of 20 voters students is selected. Thus, the final sample consists of 2,000 voters.
This is an example of:
A stratified sampling.
B multistage sampling.
C convenience sampling.
D simple random sampling.
E none of the above.