求翻译：买方应通过一家卖方可以接受的银行于装运月份前30天开立并送达卖方不可撤销即期信用证，有效至装运日后15天内在中国议付。是什么意思？

Buyer should a seller can accept the bank opened 30 days prior to the month of shipment and delivery Seller irrevocable sight letter of credit, valid until shipment within 15 days after negotiation in China.

The buyer should be adopted by a home seller can accept in the bank before shipment in 30 days and opened up to the seller to the irrevocable letters of credit, valid to shipping within 15 days after the payment in China.

The buyer should the bank which may accept through a seller previous 30 days draw up and deliver the seller in the shipping month not to be possible to abolish the immediate letter of credit, effectively to shipping in the future 15 day intrinsic Chinese negotiations.

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