--- You ought to have given them some advice.

---         , but who cared what I asked?

A．I ought to

B．So I ought

C．So it was

D．So I did

设数列{a_n}的前n项和为S_n，关于数列{a_n}有下列四个命题：

①若{a_n}既是等差数列又是等比数列，则S_n=na₁；

②若S_n=2+（-1）ⁿ，则{a_n}是等比数列；

③若S_n=an²+bn（a，b∈R），则{a_n}是等差数列；

④若S_n=pⁿ，则无论p取何值时{a_n}一定不是等比数列．

其中正确命题的序号是______．