In a sequential execution model, statements are executed serially. The fetch-decode-execute cycle runs on a single thread. This is contrasted by parallel execution model where tasks are performed on multiple threads with it not being necessary that one task must be finished before the next one starts.
The current C standard is C18, though the language has not changed significantly since C89/C90. This standard for example does not allow the use of C++ style comments (// comment) and only allows C style comments (/* comment */).
GCC allows us to specify which standard of the language we wish to use by using the -std flag.
For example, to compile a program using the C89 standard, we can enter the following command in our shell:
gcc -std=c89 file.c
This allows for great backwards-compatibility, which makes legacy software easier to maintain.