In a first order reaction, the concentration of the reactant decreases form `800 "mol dm"^(-3)` to `50 "mol dm"^(-3)` in `2 xx 10^(4) s`. The rate constant of the reaction (in `s^(-1)`) is
A. `2xx 10 ^(4)`
B. `3.45xx 10 ^(-5)`
C. `1.386xx 10 ^(-4)`
D. `2xx 10 ^(-4)`