Identify A and B in the following reaction sequence.

Bromobenzene is treated with $\mathrm{conc}. {\mathrm{HNO}}_3$ to give $2, 4, 6 \mathrm{trinitrophenol}$ by electrophilic substitution reaction mechanism, which on reaction with $\mathrm{NaOH}$ give trinitrophenol. It is also known as picric acid.