$H_2S$ gas changes a filter paper dipped in lead acetate solution into

In the following reaction,identify the nature of the product formed by the complete hydrolysis of the underlined atom at $R.T.$:
$\underline{Cl}_2O_7 + H_2O \to HClO_4$

Which one of the following statements is not correct?

$PbO_2$ reacts with concentrated $HNO_3$. The gas evolved is:

In atomic reactors,graphite is used as a

Carbon can form $p\pi-p\pi$ bond with which kind of elements?