For exports,we usually ask our buyers to open irrevocable letter

For exports,we usually ask our buyers to open irrevocable letters of credit____sellers' documentary drafts at sight
A、available by
B、available to
C、acceptable by
D、acceptable to
【正确答案】：A
【题目解析】：available by在这里是由...提供的意思；对于出口，我们通常要求买方开立卖方即期跟单汇票提供的不可撤销信用证