A researcher was interested in utilities provided by city governments. The researcher randomly selected 20 counties from a list of all counties in the U.S. From each of these counties the researcher then contacted each city government (a total of 192) and found that 12 (6.25%) of them provided electricity to their residents. The type of sample used in this example is a :
1) simple random sample
2) cluster random sample
3) stratified random sample
4) none of the above