等差数列{a­_n}的公差为d，前n项的和为S_n，当首项a₁与d变化时，a­₂+ a­₈+a­₁₁是一个定值，则下列各数中也为定值的是           

A．S₁₅                                                  B．S₁₃                                                  C．S₈                          D．S₇

A.2              B.4            C.6               D.8

A.2                   B.4                      C.6                     D.8

设等差数列的公差不为0.若是与的等比中项，则       (   )

    A.2                 B.4                 C.6                 D.8