Concept:The past continuous tense is used for an action that was already in progress when another past action suddenly occurred.Explanation:The sentence describes two past events: the thieves broke in, and the other action was happening at that same time.The action "broke into" is in the simple past tense, so the ongoing action needs the past continuous form."Were sleeping" is the correct past continuous form of "sleep" for the subject "they."The other options do not fit because "had slept" is past perfect, while "are sleeping" and "have been sleeping" are present tenses.Answer:D. were sleeping