The chemical formulas for Gypsum and Plaster of Paris are,respectively:

• A
  $CaSO_4 \cdot 2H_2O$ and $CaSO_4 \cdot 1\frac{1}{2}H_2O$
• B
  $CaSO_4$ and $CaSO_4 \cdot \frac{1}{2}H_2O$
• C
  $CaSO_4 \cdot 2H_2O$ and $CaSO_4 \cdot \frac{1}{2}H_2O$
• D
  $CaSO_4 \cdot 2H_2O$ and $CaSO_4 \cdot 3H_2O$