题目

胰岛素分子有A、B两条肽链，A链有21个氨基酸，B链有30个氨基酸，胰岛素分子中游离的氨基数目至少是（      ）  A．1个       B. 2个     C. 50个     D. 51个                                 答案：B In writing the passage, the author tends to tell the historical development of the earth．