The parametric equations of the curve $x^2+y^2-ax-by=0$ are

• A
  $x=\frac{a}{2}+\sqrt{\frac{a^2+b^2}{4}} \cos \theta, y=-\frac{b}{2}+\sqrt{\frac{a^2+b^2}{4}} \sin \theta$
• B
  $x=-\frac{a}{2}+\sqrt{\frac{a^2+b^2}{4}} \cos \theta, y=\frac{b}{2}+\sqrt{\frac{a^2+b^2}{4}} \sin \theta$
• C
  $x=-\frac{a}{2}+\sqrt{\frac{a^2+b^2}{4}} \cos \theta, y=-\frac{b}{2}+\sqrt{\frac{a^2+b^2}{4}} \sin \theta$
• D
  $x=\frac{a}{2}+\sqrt{\frac{a^2+b^2}{4}} \cos \theta, y=\frac{b}{2}+\sqrt{\frac{a^2+b^2}{4}} \sin \theta$