The ratio of the radii of the $3^{rd}$ and $5^{th}$ orbits of $He^+$ is ..... . (Atomic number of $He = 2$,$r = 0.529 \times \frac{n^2}{Z} \ \mathring{A}$)

  • A
    $9 : 25$
  • B
    $25 : 9$
  • C
    $12 : 24$
  • D
    $25 : 18$

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