A metal rod has length of 1m at ${20}^{\circ }\mathrm{C}. \alpha$ of metal is $2\times {10}^{-5}{/}^0\mathrm{C}$. The temperature at which its length will be decreased by 1mm is 
 

• A

  $-{10}^{\circ }\mathrm{C}$

• B

  $-{20}^{\circ }\mathrm{C}$

• C

  $-{30}^{\circ }\mathrm{C}$

• D

  ${30}^{\circ }\mathrm{C}$