题目

在等差数列{an}中，若a1＋a4＋a7＝39，a3＋a6＋a9＝27，则S9等于( )． A．66  B．99  C．144  D．297 答案：B----Tang is absent today. ---- _________ He is never late for work. A．Go on.            B．How come?        C．Why not?         D．Never mind