A square `S_1` encloses another square `S_2` in such a manner that each corner of `S_2` is at the mid-point of the side of `S_1` . If `A_1` is the area of `S_1` and `A_2` is the area of `S_2` , then `A_1=4\ A_2` (b) `A_1=2\ A_2` (c) `A_2=2\ A_1` (d) `A_1=A_2`