What refers to the potential audience that might receive a message through a media vehicle?

Magazine A has a cost for a full-page ad of $10,000 and a circulation figure of 5,000,000. Of these 5,000,000 subscribers, 40 percent are tennis buffs. Magazine B has a cost for a full page of $10,000 and a circulation of 3,000,000. Of the 3,000,000 subscribers 75 percent are tennis buffs. If you sold tennis racquets and wanted to compare the CPM effectiveness of these magazines, which would you choose?