The coagulation values (in mmol per litre of sol) for an $\mathrm{As}{}_2\mathrm{S}{}_3$ sol are $\mathrm{NaCl}$ = 51, $\mathrm{BaCl}{}_2$ = 0.69 and $\mathrm{AlCl}{}_3$ = 0.093. Which electrolyte has the greatest coagulating power, and about how many times greater is it than that of $\mathrm{NaCl}$?

1. A$\mathrm{AlCl}{}_3$; about 550 times
2. B$\mathrm{BaCl}{}_2$; about 74 times
3. C$\mathrm{AlCl}{}_3$; about 0.0018 times
4. D$\mathrm{NaCl}$; about 550 times