.C came first. I don’t usually use it though; I usually use .cc or .cxx, but if I’m making some tiny test source, I often use .C. I’m strongly opposed to the .cpp extension because calling C++ “CPP” leads to confusion with the preexisting (before C++) use of the initialism to refer to the C preprocessor. There’s a reason why CPPFLAGS refers to preprocessor flags and CXXFLAGS refers to C++ flags.