7. Given $P_{1}\left(x_{1}, y_{1}\right), P_{2}\left(x_{2}, y_{2}\right)$ are two points on the graph of the quadratic function $y=a x^{2}+b x+c(a b c \neq 0)$ that are symmetric with respect to the axis of symmetry. Then when $x=x_{1}+x_{2}$, the value of $y$ is ( ).
(A) 0
(B) $\frac{4 a c-b^{2}}{4 a}$
(C) $c$
(D) Cannot be determined