Which one of the following conditions needs to be satisfied for
$\phi=A x^{4}+B y^{4}+C x y^{3}$ to be considered as an Airy's stress function?
(A) $A-B=0$
(B) $A+B=0$
(C) $A-C=0$
(D) $A+C=0$
Which one of the following conditions needs to be satisfied for
$\phi=A x^{4}+B y^{4}+C x y^{3}$ to be considered as an Airy's stress function?
(A) $A-B=0$
(B) $A+B=0$
(C) $A-C=0$
(D) $A+C=0$