For the reaction A → products, concentration data were collected at 25 °C:
• t=0 s, [A] = 0.0200 M
• t=400 s, [A] = 0.0120 M
• t=800 s, [A] = 0.0060 M
Using only these values, determine whether the reaction is 0th, 1st, or 2nd order in A, and calculate the corresponding rate constant k with correct units.