题目

 Danby left word with my secretary ___he would call again in the afternoon. A. who       B. that         C. as             D. which 答案：B一个由15N标记的DNA分子，放在没有标记的环境中培养，复制4次后标记的DNA分子占DNA分子总数的（   ）A．1/4B．1/8C．1/16D．1/24