When the coordinate axes are rotated through an angle $\frac{\pi}{4}$ in the positive direction, an equation is transformed to $x^2+y^2-6 x+8 y+21=0$. Then that original equation is

• Correct answer
  A

  $x^2+y^2-7 \sqrt{2} x+\sqrt{2} y+21=0$
• B

  $\sqrt{2} x^2+\sqrt{2} y^2-7 x+y+21 \sqrt{2}=0$
• C

  $x^2+y^2-14 x+2 y+21=0$
• D

  $x^2+y^2-7 \sqrt{2} x+\sqrt{2} y+21 \sqrt{2}=0$

Hence, the correct answer is option: 'A'.