支付条款在函电中的拟定 — 国际贸易填空题
题目
某出口公司与老客户签订合同，约定凭不可撤销即期信用证付款，装运期前30天开到。业务员在给买方的函电中需补全支付条款：'Payment shall be made by an ____ letter of credit available by ____ draft at sight, reaching the seller no later than 30 days before the date of shipment.' 其中第一个空应填入的规范英文单词是 ____。