In the titration of ${\mathrm{I}}_2 \left(\mathrm{aq}\right)$ by ${\mathrm{S}}_2{\mathrm{O}}_3^{2-}$ $\left(\mathrm{aq}\right)$ using the starch indicator, the end point is indicated by:

In the balanced equation of the following reaction, the ratio of $\mathrm{a}/\mathrm{b}$ is 

 ${\mathrm{aCaCO}}_3+{\mathrm{bH}}_3{\mathrm{PO}}_4\to {\mathrm{pCa}}_3{\left({\mathrm{PO}}_4\right)}_2+{\mathrm{qCO}}_2+{\mathrm{rH}}_2\mathrm{O}$