问题：

[单选]

You are a database developer. You plan to design a database solution byusing SQL Server 2008. You are creating adataba seto support the office manager. Your  database  model has the  following  structure.

The database design has the following business requirements: An employee can be assigned more than one  task. Upon completion, thetask is deleted. When a task is deleted, the associated assignment is deleted. When an employee is nolonger available tocomple tea task, theemployee link to the assignment is replaced with a NULL value. You need to implement the business requirements to maintain data integrity.
What should you do?（）

A . Create DDL INSERT triggers on the Employee. Task and Assignment entities.
B . Create CHECK constraints on the Task ID and Employee ID attributes in the Assignment entity.
C . Create Foreign Keys constraints on the Task ID and EmployeeID attributes in the Assignment entity. Reference the Task and Employee entities respectively. Specify the appropriate On Delete action.
D . Create Foreign Keys constraints on the Task ID and Employee ID attributes in theTask and Employeeentities respectively. Reference the Assignment entity, and specify the appropriate On Delete action.