challenge: What is an example of a Single Compilation Unit in C++ programming?
  
  A. Multiple source files, including foo.cpp and bar.cpp, combined into one compilation unit
  B. A single source file, foo.cpp, with multiple classes
  C. A header file, foo.h, with implementation in bar.cpp
  D. Two separate source files, foo.cpp and bar.cpp, compiled independently
  Answer: 