Why does $NO_2$ dimerise?

Assertion $(A)$: $P_4O_{10}$ cannot be used to remove moisture from ammonia gas. Reason $(R)$: $P_4O_{10}$ reacts with $NH_3$ gas. The correct answer is:

In the ring test of $NO_{3}^{-}$ ion,$Fe^{2+}$ ion reduces nitrate ion to nitric oxide,which combines with $Fe^{2+}_{(aq)}$ ion to form a brown complex. Write the reactions involved in the formation of the brown ring.

Pure $N_2$ is obtained by:

Which of the following represents the correct order of the inert pair effect?

$(NH_4)_2Cr_2O_7$ on heating gives: