We insist that we pay you on clean collection. For larger busine

We insist that we pay you on clean collection. For larger business in future, we will accept payment __________ letter of credit.
【正确答案】：by
【题目解析】：[解析]句意：我们坚决要求用光票托收支付，以后若有更大笔的交易，我们会用信用证支付。payment by表示“用支付”。