Which of the following is metaphosphoric acid?

Concentrated nitric acid,upon long standing,turns yellow-brown due to the formation of

Two oxides of nitrogen,$NO$ and $NO_2$ are allowed to react together at $243 \ K$ to form a coloured compound of nitrogen $(X)$. When compound $(X)$ reacts with water,it yields another compound of nitrogen $(Y)$. The shape of the anion of $(Y)$ molecule is:

Which of the following products will be obtained when copper metal is reacted with $HNO_{3}$?

Nitric oxide $(NO)$ is prepared by the action of dilute $HNO_3$ on which of the following metals?

Assertion : Molecular nitrogen is less reactive than molecular oxygen.
Reason : The bond length of $N_2$ is shorter than that of oxygen.