Some C programs can be compiled in C++, yes.
C and c++ programs can work on all plate form if they do not have machine dependent code.The executable files of these can not work no different plate forms because they have machine dependent information and if machine changes the information will become meaningless .
The source or the executable?
The semi-colon converts a C++ expression into a statement.
An entry point, usually main().
Yes.
Every C plus plus program that is a main program must have the function 'main'.
Writing programs in it.
Writing computer-programs in it.
All C++ programs require an entry point and the main function provides that entry point.
The best programs are written by humans, not by softwares.
Yes.
C++ compiler, obviously, a C compiler won't do.