Regression testing focuses on finding defects after a major code change has occurred. Specifically, it seeks to uncover software regressions, or old bugs that have come back. Such regressions occur whenever software functionality that was previously working correctly stops working as intended. Typically, regressions occur as an unintended consequence of program changes, when the newly developed part of the software collides with the previously existing code. Regression testing tests the system efficiency by systematically selecting the suitable and minimum set of tests that are required to effectively cover the affected changes. Answer: A is incorrect. Unit testing is a type of testing in which each independent unit of an application is tested separately. During unit testing, a developer takes the smallest unit of an application, isolates it from the rest of the application code, and tests it to determine whether it works as expected. Unit testing is performed before integrating these independent units into modules. The most common approach to unit testing requires drivers and stubs to be written. Drivers and stubs are programs. A driver simulates a calling unit, and a stub simulates a called unit. Answer: C is incorrect. Acceptance testing is performed on the application before its implementation into the production environment. It is done either by a client or an application specialist to ensure that the software meets the requirement for which it was made. Answer: B is incorrect. Integration testing is a software testing that seeks to verify the interfaces between components against a software design. Software components may be integrated in an iterative way or all together ("big bang"). Normally the former is considered a better practice since it allows interface issues to be localized more quickly and fixed. Integration testing works to expose defects in the interfaces and interaction between the integrated components (modules). Progressively larger groups of tested software components corresponding to elements of the architectural design are integrated and tested until the software works as a system.