Topic: form select show correspond selector

now i looking for solution on how to make a drop down select form that will have difference respond to corresponding selection...<br/>
for example i make a drop down selector for occupation...<br/><br/>
when i choose information technology then it will show another selector which is specify to show only when the user choose informaion technology...that popup selector allow us to choose everything related like web design and so on...<br/><br/>
may i know how can i do it with rails ?