You use Microsoft Visual Studio 2010 and Microsoft .NET Framework 4.0 to create an application.
The application uses the ADO.NET Entity Framework to model entities.
The application includes a Customer entity along with a CustomerKey property of the Guid type as shown in the following exhibit:

You discover that when the application adds a new instance of a Customer, calling the SaveChanges method
results in the following error message: “Server generated keys are only supported for identity columns.”
You need to ensure that the application can add new Customer entities. What should you do?

A.
Add a handler for the ObjectContext.SavingChanges event.
In the event handler, set the CustomerKey value.

B.
Add a handler for the ObjectContext.ObjectMaterialized event.
In the event handler, set the CustomerKey value.

C.
Call the ObjectContext.Attach method before saving a Customer entity.

D.
Call the ObjectContext.CreateEntityKey method before saving a Customer entity.

Explanation:
SavingChanges() Event Occurs when changes are saved to the data source.
ObjectMaterialized() Event Occurs when a new entity object is created from data in the data source as part of a query or load operation.
Attach() Method Attaches an object or object graph to the object context when the object has an entity key.
CreateEntityKey() Creates the entity key for a specific object, or returns the entity key if it already exists.

ObjectContext Class
(http://msdn.microsoft.com/en-us/library/system.data.objects.objectcontext.aspx)