Which XP practice prescribes that "the code [always be] written by two programmers at one machine"?.

Pair Programming

Explanation:

In Extreme Programming XP, pair programming is a practice in which two programmers work together in a pair at one machine, when writing a code.

One of the two programmers is called the "driver" or developer who writes the code and supervises all the changes made to the program.

The other one is called an "navigator" or observer who provides ideas on how the program should be written, observes or reviews it the program, identify the issues or errors in the code, help in code simplifications.

The two developers implements the program, do coding, review the program and check each other's work. They both are required to be equally skilled in order to implement and test the code. This improves the process of the software development. By working in a pair and reviewing and testing the code together, they develop a better code.

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