59. 買賣雙方約定付款條件為不可撤銷信用狀，則在 Purchase Order 中有關 payment 的敘述，下列何者最適當？
(A)by irrevocable L/C in our favor
(B)byrevocable L/C in your favor
(C)by irrevocable L/C in your favor
(D)by revocable L/C in shipper's favor 。