Q$1.$ Choose the option that correctly answer the question.

Which of the following sentences is TRUE?

a$.$ The proper use of foreign keys is crucial to exercising data redundancy control.

b$.$ The referential integrity rule requires that every null foreign key value must reference existing primary key value.

c$.$ A table can be logically connected to another table by defining a primary key.

d$.$ It is possible to delete a row in one table whose primary key does not have a matching foreign key value in another table.