How should this be accomplished?

System Administrator has been asked to create a field on the Opportunity object that will total the invoice amount field for all the invoices associated to the Opportunity.

How should this be accomplished? – TROVARE LE RISPOSTE
A . Create a master detail relationship between invoices and opportunities and a workflow rule that will display total on the Opportunity
B . Create a lookup relationship between invoices and opportunities and a workflow rule that will display total on the Opportunity
C . Create a master detail relationship between invoices and opportunities and a Roll-Up Summary field on the Opportunity object
D . Create a lookup relationship between invoices and opportunities and a Roll-Up Summary field on the Opportunity object

Answer: C