题目

—________ do you brush your teeth? —Once a day. A . How often B . How soon C . How long D . How many 答案：A DNA分子一条链中（A＋G）／（T＋C）＝0.4，上述比例在其互补链和整个DNA分子中分别是 [　　] A．0.4和0.6 B．2.5和1 C．0.6和1 D．0.6和0.4