The slope of a common tangent to the circles $x^2+y^2=16$ and $(x-9)^2+y^2=16$ is

• A

  $\frac{8}{\sqrt{13}}$
• B

  $\frac{4}{\sqrt{13}}$
• C

  $\frac{\sqrt{17}}{8}$
• Correct answer
  D

  $\frac{8}{\sqrt{17}}$

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