What is the number of $\pi$ electrons present in pyridine?

In the process of formation of nitronium ion,nitric acid acts as

Arrange the following in increasing order of reactivity towards nitration:
$A$. $p$-xylene
$B$. bromobenzene
$C$. mesitylene
$D$. nitrobenzene
$E$. benzene
Choose the correct answer from the options given below.

In electrophilic aromatic substitution reaction,the nitro group is meta-directing because it

What is the decreasing order of reactivity amongst the following compounds towards aromatic electrophilic substitution?
$I.$ Chlorobenzene
$II.$ Benzene
$III.$ Anilinium chloride
$IV.$ Toluene

The number of benzene derivatives of the formula $C_7H_8O$ is :