A man thinks to remain in equilibrium by pushing in his hands and feet against two vertical parallel walls as the shown in the figure :  

(a) He must exert equal forces on both walls

(b) The forces of friction must be at both walls such that the torque of friction about center of gravity of man is zero

(c) The coefficients of friction between man and wall must be the same at both ends

(d) Friction must be present on at least on one wall