Which one of the following is the correct assumption made in evaluation of fully plastic moment?

• A. The upper and lower yield stresses and the modulus-of elasticity have different values in compression and tension
• B. The material is homogeneous and isotropic in both the elastic and plastic states
• C. There will be resultant axial force on the beam
• D. Some layers of the material are not free to expand and contract longitudinally and laterally under stress