* Master-detail relationships allow roll-up summary fields to aggregate data from child records. If a roll- up summary field exists on the Account object, the relationship cannot be changed to a lookup because lookup relationships do not support roll-up fields.
Why not other options?
* A: Null values in the Vendor records do not prevent the change.
* C: Custom fields can have their types changed as long as dependencies like roll-up fields are not present.
* D: Organization-wide defaults do not affect the ability to change relationships.
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Salesforce Master-Detail Relationships and Roll-Up Summary Fields