Based on common solubility rules, which compound is insoluble in water at room temperature?

• A. ${\mathrm{Ag}}_{\mathrm{Cl}}$
• B. $K_2{\mathrm{CO}}_3$
• C. ${\mathrm{Na}}_2{\mathrm{SO}}_4$
• D. ${\mathrm{NH}}_4{\mathrm{NO}}_3$

Answer: (A)