<h3>Yo ________ la música cuando el teléfono ________.</h3>
Answer: escuchaba.....sonó
Yo escuchaba la música cuando el teléfono sonó.
I listened to music when the phone rang.
Explanation: "escuchaba" is conjugated in the imperfect, as something in process without defining if it was completed and the verb "sonó" is conjugated in the preterite as a completed action that interrupts the action that was in process.
