The correct answer is option D.
i.e. graphic user interface allows users to control software operations by clicking and pointing with a mouse.
GUI is a type of interface that allows the user to interact through computer graphics.
1) You don't know if the source is reliable.
2) It is better to cite reliable sources so.
Explanation:
Refactoring consists of improving the internal structure of an existing program's source code, while preserving its external behavior.
