Given a sentence, choose either the indicative preterit or indicative imperfect tense for the verb in parenthesis.
(Ellos - quejarse) de vez en cuando.
Answer: 2) Ellos se quejaban de vez en cuando.
Explanation: The imperfect past is used in Spanish to describe an action that was habitual in the past. "De vez en cuando" means occasionally, that is to say quite habitual.
