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. $$\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}$$

If the coefficient of $${x^7}$$ in $${\left( {a{x^2} + {1 \over {2bx}}} \right)^{11}}$$ and $${x^{ - 7}}$$ in $${\left( {ax - {1 \over {3b{x^2}}}} \right)^{11}}$$ are equal, then :

The area bounded by the curves $$y=|x-1|+|x-2|$$ and $$y=3$$ is equal to :

All the letters of the word PUBLIC are written in all possible orders and these words are written as in a dictionary with serial numbers. Then the serial number of the word PUBLIC is :