question: what do birds use to crack through calcium carbonate shells?
choices.text: claws Fractures heat teeth keratin carbon twigs beaks.
choices.label: A B C D E F G H
formatted_question: what do birds use to crack through calcium carbonate shells? (A) claws (B) Fractures (C) heat (D) teeth (E) keratin (F) carbon (G) twigs (H) beaks.