题目

Catherin _______ the letter before her mother came into her bedroom.        A. has written     B. was written      C. had written      D. is writing 答案：C15．下列四个命题中，正确的是（　　）A．经过直线外一点，有且只有一条直线与这条直线平行B．同旁内角相等，两直线平行C．相等的角是对顶角D．如果两条直线都与第三条直线垂直，那么这两条直线也互相垂直