A 1 000 L water tank is dosed with KCl solution while a stirrer runs at constant speed. Conductivity sensors record the readings (in mS cm−1) at the top, middle, and bottom of the tank each minute.
Time (min) Top Middle Bottom
0 0.10 0.10 0.10
1 0.50 0.30 0.15
2 0.60 0.55 0.50
3 0.58 0.57 0.59
4 0.57 0.58 0.57
Based on these data, at which time does the tank first become effectively homogeneous with respect to KCl concentration?
a) 2min
b) 4min
c) 3min
d) 1min