题目

The program has helped out thousands of homeless children since the government ________it A. Starts  B. had started   C. will start   D. started 答案：D在数列{an}中，a1=3，且对于任意大于1的正整数n，点（an，an-1）在直线x-y-6=0上，则a3-a5+a7的值A.27B.6C.81D.9