The equation of state of some gases can be expressed as $\left(P+\frac{a}{V^{2}}\right)(V-b)=R T .$ Here, $P$ is the pressure, $V$ the volume, $T$ the absolute temperature, and $a, b, R$ are constants. The dimensions of $a$ are
(A) $\left[M L^{5} T^{-2}\right]$
(B) $\left[M L^{-1} T^{-2}\right]$
(C) $\left[M^{0} L^{3} T^{0}\right]$
(D) $\left[M^{0} L^{6} T^{0}\right]$