Show div on selection from dropdown

This might be more of an HTML general question, but I have been trying
to find a “rails” way to do this and have come up short. What I want to
do is to have a div appear if a certain selection from a drop-down is
selected or dissapeare if another option is selected. Here is what I

>Admin onselect = "alert('TEST'); document.getElementById('agency_admin_div').style.display = '';"> Agency Admin >Agent >Travel Manager >Traveler

This doesn’t work, and I have been unable to find an oppropriate rails
or html example that gives me what I want. I have done similar things
with rails helpers before but never with the “select” tag. Anybody have
any suggestions? Thanks,