Concept:The past continuous tense shows an action that was already happening when another past action interrupted it.
Explanation:The sentence has two past actions. The teacher
entered (simple past), and the boys were in the middle of discussing football at that time.
To show this ongoing past action, we use
was/were + verb-ing.
Since "the boys" is plural, the correct form is
were discussing.
Thus, the completed sentence is: "The boys
were discussing football when the teacher entered the class."
Answer:B. were discussing