The `E_(M^(3+)//M^(2+)^@` values for ` Cr`, Mn, Fe and ` Co` are ` 0.41, + 1.57 , + 0.77` and `+1, 97 V` respectively. For which one of these metals the change ub oxidation state from ` =2 ` to ` 3` is easiest :
A. `Co`
B. `Mn`
C. `Fe` and `Au`
D. `Cr`