Equation of the tangent to the curve $x^{2}+y^{2}=a^{2}$ at $\left(x_{1}, y_{1}\right)$ is (a) $x x_{1}-y y_{1}=0$ (b) $x x_{1}+y y_{1}=0$ (c) $x x_{1}-y y_{1}=a^{2}$ (d) $x x_{1}+y y_{1}=a^{2}$