Which of the following groups have an activating effect on the benzene ring?
$-OH, -COCH_3, -NO_2, -NH_2, -Cl, -Br, -NHCOCH_3, -COOH$

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:
$CH_3CH_2CH_2CH_2Br$ $\xrightarrow[\text{dry ether}]{Na} X$ $\xrightarrow[\text{10-20 atm}]{\text{Cr}_2\text{O}_3 / 773 \text{ K}} Y$ $\xrightarrow[\text{anhy. AlCl}_3]{\text{CH}_3\text{COCl}} Z$

What is obtained by the bromination of benzene in the presence of $U.V.$ light?

Heating benzene in a large excess of $80\% \, D_2SO_4$ in $D_2O$ results in what product?

Given below are two statements. One is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: $[6]-$Annulene,$[8]-$Annulene and cis$-10-$Annulene are respectively aromatic,non-aromatic and non-aromatic.
Reason $R$: Planarity is one of the requirements of aromatic systems.
In the light of the above statements,choose the most appropriate answer from the options given below.

Which of the following reactions takes place when a mixture of concentrated $HNO_3$ and $H_2SO_4$ reacts with benzene at $350 \ K$?