What could be causing this?

At Universal Containers, the Account object has a Master-Detail relationship with an Invoice custom object. The App Builder would like to change to a lookup field, but is not able to do so.

What could be causing this?
A . The invoice must have at least one Master-Detail field for reporting.
B . The Account record includes Invoice roll-up summary fields.
C . The Invoice records have existing values in the Account.
D . The Account is included in the workflow on the Invoice object.

Answer: B

Explanation:

The Account record includes Invoice roll-up summary fields. This is correct because roll-up summary fields are only available on the master object in a master-detail relationship, and changing the field type to lookup would delete the roll-up summary fields