The correct statement(s) with respect to the following sequence of reactions is/are

• Correct answer
  A

  S on treatment with $\mathrm{NaHCO}_3$ gives effervescence.
• Correct answer
  B

  $\mathbf{Q}$ decolorizes bromine water.
• Correct answer
  C

  Formation of $\mathbf{R}$ from $\mathbf{Q}$, and $\mathbf{S}$ from $\mathbf{R}$ involve carbon-oxygen bond formation.
• Correct answer
  D

  T dissolves in warm aq. KOH .

Hence, the correct answer is options: 'A', 'B', 'C', 'D'.