题目

. --You look very tired.        at all last night? --No, not really. I’m tired out now. A. Have you been sleeping           B. Were you sleeping C . Did you sleep                     D. Had you slept 答案：C9．在下列物质中，若将其中构成人体的氨基酸通过缩合而形成一条肽链，则此肽链中所含有的羧基数目正确的是（　　）A．2个B．3个C．4个D．不能确定