25. As shown in Figure 3, a square with an area of 4 is inscribed in a square with an area of 5, with each vertex of the smaller square touching the sides of the larger square, and each vertex divides the side into two segments of lengths $a$ and $b$. Then $ab=$ ( ).
(A) $\frac{1}{5}$
(B) $\frac{2}{5}$
(C) $\frac{1}{2}$
(D) 1
(E) 4
25. C.

From the problem, we know
$$
\frac{1}{2} a b \times 4=5-4 \text {. }
$$

Solving for $a b$ gives $a b=\frac{1}{2}$.