Which of the following is chemical formula of Oleum?

Q. How many of the following compounds has XX bonds i.e. single covalent bond between two same elements in their molecules :
$H_2{S}_2{O}_4,H_2{S}_2{O}_7,H_2{S}_2{O}_8,N_{2}O,N_2{O}_3,N_2{O}_4,N_2{O}_5,{\left(HP{O}_3\right)}_3$

Q. Atomicity of oleum $\left(H_2{S}_2{O}_7\right)$ is:

Q. Find the number of acid(s) which are having peroxy linkage from the following:
$H_{3}P{O}_5,H_{2}S{O}_5,H_2{S}_2{O}_7,H_2{S}_2{O}_8,HCl{O}_4$

Q. In which stage of the contact process is Oleum ($H_2{S}_2{O}_7$) obtained?