Consider the following pairs of solution which will be isotonic at the same temperature. The number of pairs of solutions is / are ___________.

A. $1 ~\mathrm{M}$ aq. $\mathrm{NaCl}$ and $2 ~\mathrm{M}$ aq. urea

B. $1 ~\mathrm{M}$ aq. $\mathrm{CaCl}_{2}$ and $1.5 ~\mathrm{M}$ aq. $\mathrm{KCl}$

C. $1.5 ~\mathrm{M}$ aq. $\mathrm{AlCl}_{3}$ and $2 ~\mathrm{M}$ aq. $\mathrm{Na}_{2} \mathrm{SO}_{4}$

D. $2.5 ~\mathrm{M}$ aq. $\mathrm{KCl}$ and $1 ~\mathrm{M}$ aq. $\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}$