Explanation

(a)
The presence of excess of sulphur makes sodium thiosulphate a useful reducing agent.
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 2Na_{2}S_{2}O_{3} + I_{3}\ \rightarrow Na_{2}S_{4}O_{6} + 2NaI$
This reaction is applied in volumetric estimation of iodine. In this reaction sodium thiosulphate acts as a reducing agent.