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. In this situation the sampling frame is :_________

a. the list of all counties in the U.S.

b. the 316 cities.

c. the 20 counties.

d. 6.25%.