The ground has three layers shown in the figure below. Calculate the effective stress at point A and $B($on the ground surface$).$

Select one:

a$.,{}_{v,A}=29\mathrm{.}34$kPa,${}_{{?}_{V,B}}=197\mathrm{.}16$kPa

b$.{}_{V,A}^{\text{'}}=29\mathrm{.}43$kPa,${}_{v,B}=79\mathrm{.}62$kPa

c$.,{}_{V,A}^{\text{'}}=-29\mathrm{.}43$kPa,${}_{V,B}=79\mathrm{.}62$kPa

d$.{}_{{?}_{v,A}^{\text{'}}}=-29\mathrm{.}43$kPa,${}_{{?}^{\text{'}},B}=197\mathrm{.}16$kPa