Which group elements from the following are called as chalcogens?

Photoconductors in xerox machines use which of the following?

The final acid product obtained during the synthesis of $H_2SO_4$ by contact process is

Describe the manufacture of $H_2SO_4$ by the contact process.

Which one of the gases dissolves in $H_2SO_4$ to give oleum?

Assertion : Both rhombic and monoclinic sulphur exist as $S_8$ but oxygen exists as $O_2$.
Reason : Oxygen forms $p\pi-p\pi$ multiple bond due to small size and small bond length but $p\pi-p\pi$ bonding is not possible in sulphur.