Why do we think that Triton was once a Kuiper Belt Object?

a. It is made of the same composition as KBOs.  

b. It has strange markings on it that cannot be explained.

c. It could not possibly have formed with the orbit it has now.

d. It has a large (20 degree) inclination to Neptune's equator.

e. It is orbiting inside the Roche Limit for Neptune.