如何在同一台机器中使用多个版本的rails

I have installed rails 3.0.10 and 2.3.5 in my machine. I wanna shuffle between them but I am unable to use 2.3.5. When I run a command, the system recognizes only 3.0.10. I have installed rails 3.0.10 and 2.3.5 in my m