what is green threading?

What is green threading means in ruby on rails , what it actually do? Is it a. " A design where fixed size pool of threads is shared around the program?" b. "when threads are emulated by virtual machine or interpreter?" or c. " where pr开发者_如何学Pythonogram run accross the multiple CPU?"

Please explain with right ans?

Green threads are threads that are emulated by a virtual machine or a runtime library instead of the operating system.