maximum shear stress

Question

If the two principal strains at a point are 1000 x$^{10^-6}$ , then the maximum shear strain is

(a) 800 x$^{10^-6}$                (b) 500 x$^{10^-6}$
(c) 1600 x $^{10^-6}$             (d) 200 x$^{10^-6}$