collection_select

<%= collection_select(:project, :id,{@arc_projects),:id,:name,options
={:prompt => "--Archived Projects--"})%> <br>
i use this code to retrive archieved project.and its works also..
  but if i want to show a dropdown which can take multiple collection
like this below code,but it will not work.
is it is possible in ruby??anybody help me??its urgent.
<%= collection_select(:project,
:id,{@arc_projects,@inactive_projects},:id,:name,options ={:prompt =>
"--Archived Projects--"})%> <br>

if it is possible then what is the write code...pls help urgent...