The correct answer is: A. they have a row in common.
A row in a database table is a set of data that describes a single entity. For example, a table of customers might have a row for each customer, with columns for the customer’s name, address, and phone number.
Two files can be joined into a third file if they have a row in common. This means that the two files must have at least one record that contains the same data for the same field. For example, if you have two files, one with a list of customers and one with a list of orders, you can join the two files if the customer ID field is the same in both files.
The other options are not necessary for two files to be joined. For example, two files can be joined even if they do not have a field in common. In this case, the join would be based on the values of other fields in the two files. Additionally, two files can be joined even if they have records with the same value in the common field. In this case, the join would create a new record for each unique combination of values in the common field.