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1.
Not credible; warrants non-belief
The idea that Oswald acted alone is disbelievable in the face of Kennedy's actions which upset the Mob and the Cubans.

If you look at the Henne family's background, its disbelievable that the family wasn't perpetrating some sort of hoax.

Shakesphere had only an elementary-school education. It's disbelievable that he wrote Hamlet.
by blogmandu October 20, 2009