Acertain first-order reaction a → b is 25% complete in 42 min at 25°C. What is its rate constant?
a) 6.8 ×10³ min⁻¹
b) 8.3 ×10 min⁻¹
c) 3.3 ×10⁻² min⁻¹
d) -3.3 ×10? min⁻¹