Let $\theta$ be the angle between the circles $S \equiv x^2+y^2+2 x-2 y+c=0$ and $S^1 \equiv x^2+y^2-6 x-8 y+9=0$. If $c$ is an integer and $\cos \theta=\frac{5}{16}$ then the radius of the circle $\mathrm{S}=0$ is

• Correct answer
  A

  2
• B

  4
• C

  3
• D

  1

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