A hydrated colorless solid (A) is water soluble and finds use in medicine as a laxative. When a solution of (A) is treated with ammonium phosphate, a white precipitate is formed. (A) gives a pink mass is cobalt nitrate test. What is (A) ?

• A

  ${\mathrm{FeSO}}_4\cdot 7{\mathrm{H}}_2\mathrm{O}$

• B

  ${\mathrm{Na}}_2{\mathrm{SO}}_4\cdot 10{\mathrm{H}}_2\mathrm{O}$

• C

  ${\mathrm{MgSO}}_4\cdot 7{\mathrm{H}}_2\mathrm{O}$

• D

  ${\mathrm{CaSO}}_4\cdot 2{\mathrm{H}}_2\mathrm{O}$