Of the interface file and the implementation file, which may be directly compiled?

a. Only the interface?
b. Only the implementation?
c. None of the above?
d. Some of the above? Which?

e)
Explanation: Only b), the implementation file, has function definitions, hence may be compiled. (While explicitly compiling a header file, such as file.h, is illegal, the implementation will necessarily #include the interface, so the interface will be compiled, but not directly.)