题目

下列能体现DNA分子特异性的是A. A/T B. A+T/G+C C. A+G/T+C D. G/C 答案：【答案】B【解析】由题文和选项的描述可知：本题考查学生对DNA分子的结构特点及其特异性的相关知识的识记和理解能力。依据碱基互补配对原则可知：在双链DNA分子中，A＝T，C＝G。可见，A/T、A+G/T+C和G/C 的比值，在所有的双链DNA分子中均等于1，不能体现DNA分子特异性，A、C、D均错误；A+T/G+C的比值， Americans eat ________ vegetables per person today as they did in 1990． [　　] A．more than twice B．as twice as many C．twice as many as D．more than twice as many