$1.$ Which of the following sentences is TRUE?a$.$ The proper use of foreign keys is crucial to exercising data redundancy control.b$.$ The referertial integrity rule requires that every null foreign key value must reference an 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