How can the brown color of $HNO_3$ be removed?

Given below are two statements $:$
Statement $I :$ Nitrogen forms oxides with $+1$ to $+5$ oxidation states due to the formation of $p\pi-p\pi$ bond with oxygen.
Statement $II :$ Nitrogen does not form halides with $+5$ oxidation state due to the absence of $d$-orbital in it.
In the light of given statements,choose the correct answer from the options given below.

Give an example to show the effect of concentration of nitric acid on the formation of oxidation products.

Which of the following statements is wrong?

What are the products obtained when ammonia is reacted with excess chlorine?

Write the structure of pyrophosphoric acid.