A stratum of soil consists of three layers of equal thickness. The permiability of top and bottom layers are k = 10-4 cm/s and that of middle layer is k = 10-3 cm/s, then the value of the horizontal coefficient of permiability KH for the entire soil layer is
1
1.2 × 10-3 cm/s
2
3.0 × 10-4 cm/s
3
4.0 × 10-4 cm/s
4
1.5 × 10-4 cm/s