What are ‘A’ and ‘B’ in the reaction

$\mathrm{HC}\equiv \mathrm{CH}+{\mathrm{H}}_2\mathrm{O}\underset{{\mathrm{H}}_2{\mathrm{SO}}_4}{\overset{{\mathrm{HgSO}}_4}{\to }}\mathrm{A}\stackrel{\mathrm{ISOMERISATION}}{\to }\mathrm{B}$

• A

  Geometrical isomers

• B

  Position isomers

• C

  Functional isomers

• D

  Optical isomers