The number of $\sigma$ and $\pi$ bonds in a molecule of benzene is

Consider the following sequence of reactions. In $Z$,the number of $sp^3$ carbons is $a$ and $sp^2$ carbons is $b$. The value of $(a+b)$ is:
$CH_3CH_2CH_2CH_2Br$ $\xrightarrow[\text{dry ether}]{Na} X$ $\xrightarrow[\text{10-20 atm}]{\text{Cr}_2\text{O}_3 / 773 \text{ K}} Y$ $\xrightarrow[\text{anhy. AlCl}_3]{\text{CH}_3\text{COCl}} Z$

Aromatic compounds burn with a sooty flame because

Total number of aromatic compounds among the following compounds is:

Among the following,the set which represents aromatic species is:

Write down the conditions for a compound to possess aromaticity.