Credible
Part of speech: adjective
Definition: believable or plausible authentic or convincing
Example sentence: A Conservative government is necessary. There is no credible alternative.
Credulous
Part of speech: adjective
Definition: Excessively ready to believe things; gullible.
Example sentence: O! Desolated Scotland, too credulous of fair speeches, and not aware of the calamities which are coming upon you! If you were to judge as I do, you would not easily put your neck under a foreign yoke.