A sand deposit comprises of three distinct horizontal layers of equal thickness. The hydraulic conductivity of the topmost layer, middle layer and the third layer is 0.002 cm/s, 0.01 cm/s, and 0.001 cm/s respectively. The equivalent value of vertical hydraulic conductivity of this sand deposit is about _______________

A

0.001 cm/s

B

0.002 cm/s

C

0.003 cm/s

D

0.004 cm/s