doubtful vs questionable

doubtful and questionable both are adjectives.

Word	Noun	Adjective	Verb	Adverb
doubtful	No	Yes	No	No
questionable	No	Yes	No	No

doubtful (adjective)	questionable (adjective)
open to doubt or suspicion	subject to question
fraught with uncertainty or doubt	able to be refuted
unsettled in mind or opinion