Step 1: Understand the time relationship. The concert ended first. Then — immediately after — the students gathered their belongings. This is a tight sequential relationship: action A triggers action B immediately. Step 2: Match the conjunction to the relationship. - 'As soon as': immediately after X, Y — perfect for instant sequence. - 'By the time': Y was already done before X finished — reverses the order here. - 'While': simultaneous actions — does not fit sequential events. - 'Until': continuous action up to a stopping point — changes the meaning. Step 3: Confirm grammar. 'As soon as the concert ended' + 'the students quickly gathered...' — past tense throughout, correct. Answer: As soon as