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 to encrypt sensitive data, while a foreign key is used to decrypt data during retrieval. 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 is used for sorting records in ascending order, while a foreign key is used for sorting records in descending order.