$4.$ A sand has a specific gravity of $2\mathrm{.}65$ and a void ratio of $0\mathrm{.}70.$ at this void ratio the angle of shearing resistance is $30\backslash$deg $.$ What is the steepest slope at which this sand can exist under the following conditions?

a$)$ The sand is perfectly dry

b$)$ The sand is submerged and there is no seepage

c$)$ The sand is saturated and there is seepage parallel to the slope

d$)$ The sand is saturated and the flow lines for seepage are horizontal.