关于函数f（x）=2^x-2^-x（x∈R）有下 * * 个结论：

①f（x）的值域为R；

②f（x）是R上的增函数；

③对任意x∈R，有f（-x）+f（x）=0成立；

其中所有正确的序号为（　　）

A．①②

B．①③

C．②③

D．①②③

The large size and roughly circular shape of the Pacific made some scientists think that it ()a hole left when the moon separated from the earth.

A. used to be

B. would have been

C. might have been

D. should have been