$n$ moles of a van der Waals' gas obeying the equation of state $\left(p+\frac{n^2 a}{V^2}\right)(V-n b)=n R T$,where $a$ and $b$ are gas-dependent constants,is made to undergo a cyclic process that is depicted by a rectangle in the $p-V$ diagram as shown below. What is the heat absorbed by the gas in one cycle?

• A
  $n(p_1-p_2)(V_2-V_1)$
• B
  $(p_1-p_2)(V_2-V_1)$
• C
  $(p_1+\frac{n^2 a}{V_1^2}-p_2-\frac{n^2 a}{V_2^2})(V_1-V_2)$
• D
  $(p_1+\frac{n^2 a}{V_1^2}-p_2-\frac{n^2 a}{V_2^2})(V_2-V_1)$