In the standardization of $Nas_2S_2O_3$ using $K_2Cr_2O_7$ by iodometry , the equivalent weight of $K_2Cr_2O_7$ is
$(a)\;\text{(molecular weight)/2} \\ (b)\;\text{(molecular weight )/6} \\(c)\;\text{(molecular weight)/3} \\(d)\;\text{Same as molecular weight}$