A man is standing on a plank which is placed on smooth horizontal surface. There is sufficient friction between feet of man and plank. Now man starts running over plank, correct statement is/are:

At contact point of man and plank, the relative motion is zero. As plank moves backward, man's foot also moves backwards, but friction on man acts forward. Hence work done by friction on man is negative. But with respect to plank, foot will be at rest, hence no work done by friction on man w.r.t. plank.