题目

在中，，，，则等于(    )                                                                                                                                                                                                                     A.          B.          C. 或      D. 以上答案都不对12．设数列{an}的前n项和为Sn，Sn=2an-2．（Ⅰ）求数列{an}的通项公式；（Ⅱ）数列{bn-an}是首项为1，公差为3的等差数列，求数列{bn}的前n项和Tn．