The reactant concentration in a zero-order reaction was 8.00 × 10⁻² M after 180 s and 4.00 × 10⁻² M after 400 s. What is the rate constant for this reaction?
a) 2.22 × 10⁻⁴ M/s
b) 1.11 × 10⁻⁴ M/s
c) 5.56 × 10⁻⁴ M/s
d) 2.78 × 10⁻⁴ M/s