题目

---Your grandfather still seems very energetic, although he is already in his seventies now. ---Yes, he _______in the army for three years when he was young. A． would serve             B． served C． had served               D． has served 答案：B在生理pH条件下，下列哪种氨基酸带正电荷？ A．丙氨酸 B．酪氨酸 C．赖氨酸 D．蛋氨酸 E．异亮氨酸