How to keep ember bootstrap accordion expanded by default?

what attribute should I pass to keep bootstrap accordion open.

<BsAccordion as |acc|>
  <acc.item @value=1 @title="First item">
    <p>Lorem ipsum...</p>
    <button >
      Next
    </button>
  </acc.item>
  <acc.item @value=2 @title="Second item">
    <p>Lorem ipsum...</p>
  </acc.item>
  <acc.item @value=3 @title="Third item">
    <p>Lorem ipsum...</p>
  </acc.item>
</BsAccordion>