Let the angles $\mathrm{A}, \mathrm{B}, \mathrm{C}$ of a triangle ABC be in arithmetic progression. If the exradii $r_1, r_2, r_3$ of triangle ABC satisfy the condition $r_3^2=r_1 r_2+r_2 r_3+r_3 r_1$, then $b=$

• A

  $\frac{2 a}{\sqrt{3}}$
• B

  $\sqrt{2} a$
• Correct answer
  C

  $\sqrt{3} a$
• D

  a

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