Before revision: During the 1990s, a series of videos were mass-marketed to the public; by 1999 an estimated 1.5 million sets of videos had been sold on the back of a frequently aired television infomercial .
After revision: During the 1990s, a series of videos was mass-marketed to the public; by 1999 an estimated 1.5 million sets of videos had been sold by frequently-aired television infomercials, .