9. For non-empty sets $A$ and $B$, define the operation:
$A \oplus B=\{x \mid x \in A \cup B$, and $x \notin A \cap B\}$.
Given two open intervals $M=(a, b), N=(c, 
d)$, where $a, b, c, d$ satisfy
$$
a+b<c+d, ab=cd<0 .
$$

Then $M \oplus N=(\quad)$.
(A) $(a, b) \cup(c, d)$
(B) $(a, c) \cup(b, d)$
$(\mathrm{C})(a, d) \cup(b, c)$
(D) $(c, a) \cup(d, b)$
9. B.

Let $a b=c d=t(t<0)$. Then
$$
a<0<b, c<0<d \text {. }
$$

Construct the functions
$$
\begin{array}{l}
f(x)=x^{2}-(a+b) x+t, \\
g(x)=x^{2}-(c+d) x+t .
\end{array}
$$

Then $a, b$ are the two roots of the equation $f(x)=0$, and $c, d$ are the two roots of the equation $g(x)=0$.
$$
\begin{array}{l}
\text { Also, } f(c)=c^{2}-(a+b) c+c d \\
=c[(c+d)-(a+b)]<0,
\end{array}
$$

Thus, $a<c<d$.
Similarly, $c<b<d$.
Therefore, $a<c<b<d$.
Hence, $M \oplus N=(a, c) \cup(b, d)$.