正答:③
B の "Maybe I can finish early and go" が決め手。「早く終わったら行ける"かも"」と条件付きの発言なので、③が最適。①は"really want to"と矛盾、②は本文に根拠なし、④はチケットを買うのはA。
SCRIPTA: Hey, are you going to the concert on Saturday?
B: I really want to, but I have to study for my math exam on Monday.
A: Oh, that's tough. The tickets are almost sold out.
B: Actually, my sister said she'd help me study tonight. Maybe I can finish early and go.
A: Great! Let me know soon so I can buy two tickets.