5. Let non-zero complex numbers $x, y$ satisfy $x^{2}+x y+y^{2}=0$, then the value of the algebraic expression $\left(\frac{x}{x+y}\right)^{1990}$ $+\left(\frac{y}{x+y}\right)^{1990}$ is
(A) $2^{-1989}$;
(B) -1 ;
(C) 1 ;
(D) None of the above answers is correct.