A metal wire has radius $r$ and carries a constant current $I$. It is thermally insulated and its resistivity is temperature independent. The coefficient of thermal expansion is negligible. The rise in temperature of the wire per unit time is found to be proportional to $r^n$. Here $n$ is

• A

  $0$
• B

  $-2$
• Correct answer
  C

  $-4$
• D

  $-6$

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