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 this to a lookup field, but is not able to do so.

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

Answer: D