sentences:  The second was the cassette single where the song is released on an audio tape - the BPI began recording their sales this year, but they would not become eligible until 1989.Both formats would dominate the chart throughout the 1990s.Synthpop bands Pet Shop Boys and Erasure continued their success from their breakthrough year in 1986, with the Pet Shop Boys achieving their second number 1 single ("It's a Sin") in the summer, and two more top ten singles with "What Have I Done to Deserve This?