In $\triangle A B C$, if $r=3$ and $R=5$ then $\frac{1}{a b}+\frac{1}{b c}+\frac{1}{c a}=$

• Correct answer
  A

  $\frac{1}{30}$
• B

  $\frac{12}{15}$
• C

  $\frac{1}{15}$
• D

  $\frac{5}{36}$

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