Please help, i don't know why the critical region is S/C, where S is square and C is circle. Please help clarify.

(6) Suppose X1 and X2 are iid samples and consider the null hypothesis {Hg : X:- N MN, 1) versus the alternative 9131 t X;- N UH], 2] for 2' = 11 2. (a) Show that the critical region of a likelihood ratio test is of the form 3 \\ C, where S is a square and C is a circle. (1)) Determine the size and power of the test gt: for which the radius of the circle equals the side length of the square. (e) Determine the power of the test if) for which the size is 0.05