Which of the following pairs can be distinguished by the action of heat?

• A

  ${\mathrm{Na}}_2{\mathrm{CO}}_3\text{ and }{\mathrm{CaCO}}_3$

• B

  ${\mathrm{K}}_2{\mathrm{CO}}_3\text{ and }{\mathrm{MgCO}}_3$

• C

  $\mathrm{Ca}{\left({\mathrm{NO}}_3\right)}_2\text{ and Li }{\mathrm{NO}}_3$

• D

  ${\mathrm{MgCl}}_2\cdot 6{\mathrm{H}}_2\mathrm{O}\text{ and }{\mathrm{CaCl}}_2\cdot 6{\mathrm{H}}_2\mathrm{O}$