In a relational database, what is the purpose of a primary key and a foreign key?

a$.$ Primary key is used for sorting records in descending order, while a foreign key is used for sorting records in ascending order.

b$.$ Primary key ensures data integrity by preventing duplicate values in a table, while a foreign key defines relationships between tables.

c$.$ Primary key is used is used for sorting records in ascending order, while a foreign key is used for sorting records in descending order.

d$.$ Primary key is a unique identifier for a table, while a foreign key is used to store redundant information for backup purposes.

e$.$ Primary key is used to encrypt sensitive data, while a foreign key is used to decrypt data during retrieval.