题目

From the address________on the envelope，we see that the letter is from America. A . marking B . marked C . to be marked D . having marked 答案：B如果a∥b，b∥c，那么a∥c，这个推理的依据是（　　）A．等量代换B．平行线的定义C．经过直线外一点，有且只有一条直线与已知直线平行D．平行于同一直线的两直线平行