题目

—This returned scholar has become one of the top experts in this field.    —Yes, I know him very well. He ______ for ten years at an institute in the USA. A. has worked                   B. has been working            C. worked   D. had worked  答案：C脑啡肽是一种具有镇痛作用的药物，它的基本组成单位是氨基酸。下面是脑啡肽的结构简式，形成这条肽链的氨基酸分子数以及缩合过程中生成的水分子数分别是（ ）A. 4和3 B. 6和5 C. 3和2 D. 5和4