6. They ___ watch TV two days ago. a) don't b) did c) didn't

The correct answer is c) didn't.

The sentence is a negative statement about a past event ('two days ago'). The auxiliary verb 'did' combined with 'not' (didn't) is used to form the negative past tense. The main verb 'watch' remains in its base form.

Ответ: c) didn't