A First Order Reaction, with Rate Constant K = 3

A first order reaction, with rate constant k = 3.25 × 10^-5 s^-1, at 200 °C, is followed experimentally over a period of 1 hour. The initial concentration of the reactant was 0.0280 mol dm^-3. What was the concentration of the reactant after one hour?

A) 0.0279 mol dm^-3
B) 0.0249 mol dm^-3
C) 0.0278 mol dm^-3
D) -0.089 mol dm^-3