For the sentence "mientras vosotros ________ el postre, yo hice el café", why is "estuvisteis comiendo" incorrect? The lesson seems to suggest that it should be correct.
For example, the lesson says this: When the ongoing past action is inside a completed timeframe in the past. See how all examples above contain a time phrase: "toda la noche", "de 6 a 7 de la tarde", "el sábado pasado". In the quiz question, the completed timeframe is "while you were having dessert". Isn't that a similar time frame to "toda la noche"?
Also, the lesson says this: We don't know when the "eating" started, but the end is expressed with "hasta que acabamos con toda la comida". This may sound as if that is an interrupting action and may lead you to wrongly think you need to use El Imperfecto Progresivo [estaba+present participle]. Likewise, in the quiz question, we know that "having dessert" has a beginning and an end, although we don't know exactly when.
Gracias!
