A first order reaction is carried out with an initial concentration of `10` mol litre and `80%` of the reactant changes into product in `10 sec`. Now if the same reaction is carried out with an intial concentration of `5` mol per litre the percentage of the reactant changing to the product in `10` sec is:
A. `40`
B. `80`
C. `60`
D. `50`
