$A$ : In acute constipation,purgatives containing magnesium salts are generally used.
$R$ : The osmotic effect of $Mg^{2+}$ in the intestinal lumen prevents water reabsorption from the intestine. $Mg^{2+}$ increases the solute concentration in the intestinal lumen because $Mg^{2+}$ is absorbed very slowly.

• A
  Assertion and Reason both are correct and Reason is the correct explanation of Assertion.
• B
  Assertion and Reason both are correct but Reason is not the correct explanation of Assertion.
• C
  Assertion is correct,but Reason is incorrect.
• D
  Both Assertion and Reason are incorrect.