题目

 It’s difficult for us to learn a lesson in life _______ we’ve actually had that lesson.        A. after                            B. since                           C. until                             D. when 答案：C13．在等差数列{an}和等比数列{bn}中，已知a1=-8，a2=-2，b1=1，b2=2，那么满足an=bn的n的所有取值构成的集合是{3，5}．