Objections to prior bad acts are made on several grounds-- usually that the evidence is irrelevant and highly prejudicial under Rule 403, or inadmissible character evidence under Rule 404(a). Rule 404(b) is generally invoked as a response to an objection, e.g., that the evidence of a prior crime is not excluded by the character evidence rule, but is admissible for one of the reasons listed in 404(b).
Problem 5-66, page 181.