5. Let $a, b, c, d$ be distinct real numbers, and
$$
\begin{array}{l}
\left(a^{2}-c^{2}\right)\left(a^{2}-d^{2}\right)=1, \\
\left(b^{2}-c^{2}\right)\left(b^{2}-d^{2}\right)=1 .
\end{array}
$$

Then $a^{2} b^{2}-c^{2} d^{2}=(\quad)$.
(A)0
(B) -1
(C) 1
(D) Cannot be determined
5. B.
$a^{2}, b^{2}$ are the roots of the equation $\left(x-c^{2}\right)\left(x-d^{2}\right)=1$. Expanding it, we get $x^{2}-\left(c^{2}+d^{2}\right) x+c^{2} d^{2}-1=0$. By the relationship between roots and coefficients, we have $a^{2} b^{2}=c^{2} d^{2}-1$. Therefore, $a^{2} b^{2}-c^{2} d^{2}=-1$.