A Frank$-$Read source is operating in a fcc crystal which has a shear modulus of $50$ GPa $.$ If the dislocation pinning points are $2$ microns apart and the magnitude of the dislocation burgers vector is $0\mathrm{.}4$ nm $,$ determine the shear stress, in units of MPa $,$ required to operate the source.

$1$

$10$

$100$

$1000$