What could be causing this?

AtUniversal 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