交易条件修改的表达方式 — 国际贸易选择题

外贸函电中，卖方收到买方还盘后，希望买方将订单量由 500 件提高到 1000 件方可接受原报价。下列哪一句最恰当地表达了这一交易条件修改？ A. We regret to inform you that our price has been lowered to USD 10 per piece. B. We can accept your counter-offer only if you increase your order to 1,000 pieces. C. Thank you for your order of 500 pieces; we have shipped the goods today. D. Please note that the quantity remains unchanged at 500 pieces as quoted.