Multiple selection in select box in Rhtml

select_tag('letters[]', options_for_select(['A','B','C','D'], 'A'),
:size => 5, :multiple => true)

You need to add the [] to the name to get back an array of selected
values.

That works great! Thanks!! But what if I need to display the form
with more then one value already selected?
I tried

  select_tag('letters[]',
options_for_select(['A','B','C','D'],'A','B'), :size =>5, :multiple =>
true)

but that raised the following error: "Wrong number of arguments (3
for 2)".

Any suggestions?