Commercials

Do advertisers get to choose during what shows their advetisments are seen? Or just choose the general time and channel? For example, lots of meds4home and scooters america and such get aired during The Price Is Right. The reason I'm wondering is cause I was just watching BBC Planet Earth, a beautiful nature documentary, and there seemed to be a strangely high number of McDonalds commercials. Somehow I didn't think nature documentary watchers were McDonalds target demographic... I'd expect to see more along the lines of yogurt commercials.