Schmid's law

$1.$The term cos phi cos lambda in the expression of the resolved shear stress is known as the Schmid factor. Why is the Schmid factor the crucial criterion which decides which slip system is activated?

$2.$A silver single crystal undergoes a tensile test with an applied force in $[123]$ direction. Considering all slip systems in the crystal structure, determine:

a$)$The angles phi and lambda for all slip systems possible.

b$)$The slip system that is activated $($as the tensile test starts$).$

$3.$If the critical resolved shear stress is $20\mathrm{.}7$ MPa $,$ will an applied stress of $45$ MPa cause the single crystal to yield? $($Assume the initially activated slip system only$).$

$4.$ How long will the activated slip system remain active? What will happen then?