18.83 As shown in the figure, a red cross of equal width (symmetric about the diagonals) is on a square flag, with a small blue square in the center, and the rest is white. If the entire cross (including red and blue) occupies 36% of the flag's area, what is the percentage of the blue square's area relative to the flag's area?
(A) 0.5 .
(B) 1 .
(C) 2 .
(D) 3 .
(E) 6 .
(40th American High School Mathematics Examination, 1989)
[Solution] Let the side length of the square be 10, then its area is 100. According to the problem, we have
$$
S_{\text {white }}=100 \cdot(1-36 \%)=64 \text {, }
$$

and the four small white triangles can be combined to form a square, whose side length is $\sqrt{64}=8$.
Thus, as shown in the figure, we have
$$
A B=\frac{1}{2}(10-8)=1 \text {, }
$$

so $E B=\sqrt{1^{2}+1^{2}}=\sqrt{2}$
Therefore,
$$
S_{\text {blue }}=(\sqrt{2})^{2}=2 \text {. }
$$

Hence, the answer is $(C)$.