By process of elimination, I selected estuvieron as being the correct answer; however, my preference would have been estaban.
See the example given below:
Mario estaba de camarero en un restaurante.
Mario was working as a waiter in a restaurant.
Here is the question with the answer:
Mis hijos _estuvieron_
de vendedores en una tienda.
My children were working as shop assistants in a store.
Mis hijos _estaban_ de vendedores en una tienda.
Would you explain the reason for the answer being the preterite and not the imperfect, please.