Consider the following pairs of solution which will be isotonic at the same temperature. The number of pairs of solutions is/are……… (a) 1 M aq. NaCl and 2 M aq. Urea (b) 1 M aq. $\mathrm{CaCl}_2$ and 1.5 M aq. KCl (c) 1.5 M aq. $\mathrm{AlCl}_3$ and 2 M aq. $\mathrm{Na}_2 \mathrm{SO}_4$ (d) 2.5 M aq. KCl and 1 M aq. $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$