$(4)(2$ points$)$ In calculating seepage in soils, the equation $q=k*h*(\frac{N_f}{N_d})$ is used. Under which of the following conditions, this equation may NOT be used?

$($A$)$ The soil is heterogeneous.

$($B$)$ The soil is homogeneous.

$($C$)$ The soil is isotropic.

$($D$)$ The flow lines and the equipotential lines must be perpendicular to each other.