The constant b in the equation in terms of specific volume at critical point $V_{c}$ is equal to

Question

Van der walls equaltion of state is given by (P+ a/V² ) (V-b)= RT. The constant b in the equation in terms of specific volume at critical point $V_{c}$ is equal to                                           

(a) V_c/3                        (b) 2V_c

(c) 3V_c                         (d)8a /(27 V_c r)