Form helper and select_tag

I can generage select-tag easily and it works fine:

<%= select_tag "category",
options_from_collection_for_select(@categories, "id", "name"), :multiple
=> true, :include_blank => true %>

But how can I make "selected" some values?

@selected_categories = Array[1, 3]

So id 1 and 3 should be: <option selected="selected">

How can I do it?

Start by reading the documentation for
options_from_collection_for_select

Fred