6. Given points $A(-7,0), B(7,0), C(2,-12)$. If one focus of an ellipse is $C$, and the ellipse passes through points $A$ and $B$, the locus of the other focus of the ellipse is ( ).
(A) Hyperbola
(B) Ellipse
(C) Part of an ellipse
(D) Part of a hyperbola
6. (D).

Let the other focus of the ellipse be $F(x, y)$.
Since $A$ and $B$ are points on the ellipse, by the definition of an ellipse, we have $|A C|+|A F|=|B C|+|B F|$,
thus $|B F|-|A F|=|A C|-|B C|$.
Given $|A C|=15,|B C|=13$, we get $|B F|-|A F|=2$, hence the locus of point $F$ is part of a hyperbola.