15. When I stayed in a shared self-catered accommodation during studying abroad, my roommate frequently offered: “_____” (A) If you help me with the shopping, I will help you with your paper. (B) If you helped me with the shopping, I would help you with your paper. (C) If you have helped me with the shopping, I would have helped you with your paper. (D) If you are helping me with the shopping, I will be helping you with your paper.