$\text { The cartesian equation of the curve given by } x=6 \cos \theta, \quad y=6 \sin \theta \text { is }$ (A) $x^{2}+y^{2}=36$ (B) $x^{2}+y^{2}=5$ (C) $x^{2}+y^{2}=25$ (D) $x^{2}+y^{2}=6$