A. A beam in which extreme fibre stresses are same at all cross-sections along the length of the beam. B. A beam in which the moment of inertia about the axis of bending is constant at all cross-sections of the beam. C. A beam in which the distribution of bending stress across the depth of cross-section is uniform of all cross-sections of the beam. D. A beam in which the bending stress is uniform at the maximum bending moment cross-section.