Owners of a radio station want to obtain information on the radio listening habits in their market area. The market area contains one city of population 150,000, another city of 40,000, and three towns of about 6,000 inhabitants each. The station suspects the listening habits may be different in larger and smaller cities, and in the rural areas, so it splits the market area up accordingly to survey. Which of the following sampling designs best represents the type of survey design described?