A metal rod of length $$L$$ at temperature $$T$$, when heated to temperature $$T'$$, expands to new length $$L'$$. These quantities are related as $$L'=L(1+\alpha[T'-T])$$ where $$\alpha$$ is a constant for that material and called as coefficient of linear expansion. Correct SI unit of $$\alpha$$ is

A

$$\text{m}\,\text{K}^{-1}$$

B

$$\text{m}\,\text{K}$$

C

$$\text{K}^{-1}$$

D

$$\alpha$$ is a pure number