设线性无关的函数 $y_{1}, y_{2}, y_{3}$ 都是二阶非齐次线性方程 $y^{\prime \prime}+p(x) y^{\prime}+q(x) y=f(x)$ 的解, $C_{1}, C_{2}$ 是任意常数, 则该非齐次方程的通解是 ( )
A. $C_{1} y_{1}+C_{2} y_{2}+y_{3}$.
B. $C_{1} y_{1}+C_{2} y_{2}-\left(C_{1}+C_{2}\right) y_{3}$.
C. $C_{1} y_{1}+C_{2} y_{2}-\left(1-C_{1}-C_{2}\right) y_{3}$.
D. $C_{1} y_{1}+C_{2} y_{2}+\left(1-C_{1}-C_{2}\right) y_{3}$.