$13.$ For a shaft subjected to both bending and torsion, the equivalent stress should not exceed:

$-$ A$)$ The yield strength of the material

$-$ B$)$ Half the yield strength

$-$ C$)$ Twice the yield strength

$-$ D$)$ The ultimate tensile strength

$14.$ The design of a shaft based on stiffness primarily ensures:

$-$ A$)$ Minimum cost

$-$ B$)$ Minimum weight

$-$ C$)$ Acceptable deflection

$-$ D$)$ Maximum strength