A clay liner has a hydraulic conductivity of 1.55 × 10−8 cm/s, a porosity of 0.2, and a hydraulic head of 0.15 m. The thickness of the liner to yield a breakthrough time of 20 years is most nearly?

a. 0.4 m
b. 0.5 m
c. 0.6 m
d. 0.7 m