For two dimensional case, for both plane stress as well as plain strain case the compatibility equation is _______

• A. ( frac{ ^2 _x}{ z^2} + frac{ ^2 _z}{ x^2} = frac{ ^2 _{xz}}{ x z} )
• B. ( frac{ ^2 _z}{ z^2} + frac{ ^2 _y}{ x^2} = frac{ ^2 _{zy}}{ z y} )
• C. ( frac{ ^2 _x}{ y^2} + frac{ ^2 _y}{ x^2} = frac{ ^2 _{xy}}{ x y} )
• D. ( frac{ ^2 _z}{ z^2} + frac{ ^2 _y}{ x^2} =0 )