The state of stress in 3-dimension is given by the following determinant, the coefficient of 2 is an invariant quantity which is equal to ________ ( begin{vmatrix} sigma- sigma x & - tau yx & - tau zx - tau xy & sigma- sigma y & - tau zy - tau xz & - tau yz & sigma- sigma z end{vmatrix} )

• A. x+ y+ z
• B. x y+ y z+ z x- xy2- yz2- zx2
• C. x y z+2 xy yz zx- x xy2 - y yz2 - z zx2
• D. 1