Which two reasons could be preventing this relationship conversion?

Universal Containers (UC) has a custom Invoice object and a custom InvoiceLine Item object. TTie Invoice Line Item object has a lookup relationship to the Invoice. UC would like to convert the lookup relationship to a master-detail relationship but is unable to do so.

Which two reasons could be preventing this relationship conversion?

Choose 2 answers
A . Custom objects are unable to be on the detail side of a master-detail relationship.
B. There are already two master-detail relationships on the Invoice Line Item.
C. Invoice Line Item records exist without having the Invoice lookup field populated.
D. There is a roll-up summary field on the Invoice object.

Answer: B,C