Consider an element of infinitesimal volume, in a fluid at equilibrium. In this situation we can say that:

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(c)  the forces that the adjacent elements exert of it are parallel to the separating surfaces

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(e)  the forces exerted by adjacent elements of it are perpendicular to the separating surfaces

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(b)  the forces exerted by adjacent elements of it are tangent to the separating surfaces

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(d)  the forces which the adjacent elements exert of it are necessarily zero

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(a)  the forces exerted by adjacent elements of it are both perpendicular and tangent to the separating surfaces

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