The correct answer is: D. Both a and b
A database management system (DBMS) is a software application that allows users to create and manage databases. A database is a collection of data that is organized in a way that makes it easy to retrieve and use. The physical schema is the physical structure of the database, which includes the way that the data is stored on disk. The sub schemas are the logical views of the database, which are created by users to represent the data in a way that is meaningful to them.
In order to use a DBMS, it is important to understand both the physical schema and the sub schemas. The physical schema is important because it determines how the data is stored and accessed. The sub schemas are important because they determine how the data is presented to users.
For example, a company might have a database that stores information about its customers. The physical schema of this database might include tables for customer records, order records, and product records. The sub schemas of this database might include a sub schema for sales representatives, which would allow them to view information about their customers and orders; a sub schema for customers, which would allow them to view their account information; and a sub schema for management, which would allow them to view all of the data in the database.
In order to use this database, sales representatives would need to understand the sales representative sub schema. Customers would need to understand the customer sub schema. And management would need to understand all of the sub schemas.
Therefore, the correct answer is: D. Both a and b