Rails simple_form - Align radio buttons vertically

I have the following code:

<%= simple_form_for @comment do |f| %>
    <%= f.input :comment, label: 'Phrase you would like to add:' %>
    <%= f.collection_radio_buttons :options, [[true, 'Positive'] ,[false, 'Negative']], :first, :last, label: 'Emotion' %>
<% end %>

This works fine, but the radio buttons in collection_radio_buttons are side by side. How can I stack the radio buttons vertically on top of each other?