Which two reasons could be preventing this relationship conversion?

Universal Containers (UC) has a custom Invoice object and a custom Invoice Line 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

Explanation:

The two reasons that could be preventing this relationship conversion are that there are already two

master-detail relationships on the Invoice Line Item and that Invoice Line Item records exist without having the Invoice lookup field populated. A custom object can have up to two master-detail relationships, so adding another one would exceed the limit. Also, a master-detail relationship requires that the detail records have a value in the master lookup field, so any records with a blank Invoice lookup field would prevent the conversion.