题目

 Paris is one of _______________________in the world.        A. beautiful city           B. the most beautiful city       C. the most beautiful cities     D. more beautiful cities 答案：C已知关于x的方程x2+（1+a）x+1+a+b=0（a，b∈R）的两根分别为x1、x2，且0＜x1＜1＜x2，则的取值范围是    ．