A square with area 4 is inscribed in a square with area 5 , with one vertex of the smaller square on each side of the larger square. A vertex of the smaller square divides a side of the larger square into two segments, one of length a and the other of length b. What is the value of a b ?
Answer Choices
A. \dfrac{1}{5}
B. \dfrac{2}{5}
C. \dfrac{1}{2}
D. 1
E. 4