—Do you mind me sitting beside you?

—        . The seat is for my sister.

大象可以用人类听不到的声音进行交流，其实质是（    ）

阅读理解
    Mr. Black is American, but he is in Canada now. He is a tall man with funny glasses. He works in a big city but lives in a town far away. He lives on the twentieth floor of a building. Every morning he gets up early. Then he gets into the lift and it takes him down to the first floor. He runs to the bus stop and catches the early bus. It usually takes him about two hours to get to the factory by bus. Then he gets off the bus and walks into the workshop(车间) quickly. He starts his work at eight in the morning.

已知a＞0， ${\left(\frac{a}{\sqrt{x}}-x\right)}^6$ 展开式的常数项为15，则 ${\displaystyle \int {}_{-a}^a}\left(\sqrt{1-x^2}+\sin 2x\right)dx$ =____

设数列{a_n}的前n项和为S_n，关于数列{a_n}有下列四个结论：

①若数列{a_n}既是等差数列又是等比数列，则S_n=na₁；

②若S_n=2^n﹣1，则数列{a_n}是等比数列；

③若S_n=an²+bn（a，b∈R），则数列{a_n}是等差数列；

④若S_n=an（a∈R），则数列{a_n}既是等差数列又是等比数列．

其中正确结论的序号是____．