题目

23．下列物质中，找出构成生物体蛋白质的氨基酸，若这些氨基酸脱水缩合形成一条肽链，则所形成的肽链中羧基的数目是(   ) A.2个 B.3个 C.4个 D.至少3个 答案：D— Why did Karen ______ your invitation?— Because she had to look after her little brother.A. turn down B. write down C. look for