A man balances himself in a horizontal position by pushing his hands and feet against two parallel walls. His centre of mass lies midway between the walls. The coefficients of friction at the walls are equal. Which of the following is not correct?
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(a) He exerts equal forces on the walls.
(b) He exerts only horizontal forces on the walls.
(c) The forces of friction at the walls are equal.
(d) The forces exerted by the walls on him are not horizontal.