Start by drawing some pictures so that you understand what is going on.

is a circle with center at (0, 0) and radius . is a circle with center at (0, 0) and radius .

is the region on and between those two circles. When k= 1, that is the "washer" between radius 1 and radius . When k= 2, that is the "washer" between radius and . Notice that the first set is completely contained in the second so the union of the two sets is just the second set. What does go to as k goes to infinity? What does go to as k goes to infinity?