(b)
When calcium carbide reacts with water, ethyne is formed.
$CaC_{2} + 2H_{2}O\overset{\rightarrow}{- Ca(OH)_{2}}C_{2}H_{2}$
Calcium carbide ethyne
$\underset{(ii)H_{2}O/H^{+}}{\overset{\ \ \ \ \ (i)O_{3}\ \ \ \ \ \ \ }{\rightarrow}}HCOOH + HCOOH$
Formic Formic
acid acid