题目

一只普通白炽灯，不通电时灯丝的电阻为R1；正常发光时灯丝的电阻为R2。比较R1和R2的大小，应是（     ）A．R1＞R2      B．R1＜R2       C．R1＝R2      D．条件不足，无法判断  答案：B解析:略 This is        ruler and that is        eraser. A．a, an            B．an, a           C．a, a