下列关于信用证特点的说法，正确的是( )。

A．信用证付款是一种银行信用

B．信用证是完全独立于合同之外的一种自足性文件

C．信用证结算按其资金流向与支付工具传送方向，属于顺汇

D．信用证项下付款是一种单据的买卖

Woman: Bill, I want to have a few words with you about your performance in class lately.

Man: I know I’ve gone down. I just haven’t been studying as much as I ought to.

Question: What is Bill’s problem().

A. He doesn’t like to perform in class.

B. He doesn’t work hard enough.

C. He has gone away lately.

D. He feels depressed.