Presence of which reagent will affect the reversibility of the following reaction, and change it to a irreversible reaction :

CH₄ + I₂ $$\mathrel{\mathop{\kern0pt\rightleftharpoons} \limits_{{\mathop{\rm Re}\nolimits} versible}^{hv}} $$ CH₃ $$-$$ I + HI

An organic compound 'A' C₄H₈ on treatment with KMnO₄/H⁺ yields compound 'B' C₃H₆O.

Compound 'A' also yields compound 'B' an ozonolysis. Compound 'A' is :

Metallic sodium does not react normally with :

Benzene on nitration gives nitrobenzene in presence of HNO₃ and H₂SO₄ mixture, where :