A single crystal of a metal is oriented for a tensile test such that its slip plane normal makes an angle of 66.4° with the tensile axis. Three possible slip directions make angles of 30°, 48°, and 78° with the same tensile axis.(a) Which of these three slip directions is most favored? (b) If plastic deformation begins at a tensile stress of 2.5 MPa (355 psi), determine the critical resolved shear stress for zinc.