A consolidated drained test was conducted on a soil sample. At the failure, the major

and minor principal stresses were ${}_{1f}=139$kPa and ${}_{3f}=43$kPa. The frictional

angle of the soil was found to be $32$ degrees and the cohesion is $5$kPa. What was

the shear stress on the failure plane of the sample at failure moment? Unit: kPa

$91$

$53$

$41$

$65$