题目

The decorations were so lovely that they _____ in a short time.     A. sold out    B. has been sold out    C. were sold out     D. were selling out 答案：C设正项等比数列{an}的首项a1＝，前n项和为Sn，且210S30－(210＋1)S20＋S10＝0，则an＝_        ．