Decomposition is a fundamental principle of computational thinking. It involves breaking down a complex problem or system into smaller, more manageable, and understandable sub-problems. Each sub-problem can then be solved independently, and the solutions can be combined to solve the original complex problem. This approach simplifies the development, testing, and maintenance process.

B. Turning off a computer is a user action related to hardware operation, not a problem-solving methodology.

C. Combining files is an act of aggregation or archiving, which is the conceptual opposite of decomposition.

D. Replacing variables with constants is a specific coding practice, not the broad problem-solving strategy of decomposition.

1. Wing, J. M. (2006). Computational thinking. Communications of the ACM, 49(3), 33-35. (Section: "The concepts of computational thinking", para. 2). https://doi.org/10.1145/1118178.1118215

2. Grover, S., & Pea, R. (2013). Computational Thinking in K–12: A Review of the State of the Field. Educational Researcher, 42(1), 38-43. (Section: "What is Computational Thinking?", p. 39). https://doi.org/10.3102/0013189X12463051

3. MIT OpenCourseWare. (2016). 6.0001 Introduction to Computer Science and Programming in Python, Fall 2016. Lecture 1: What is Computation? Notes. (The concept is introduced as a core problem-solving technique).