Elephants are often observed licking clay from mud holes. There is ample evidence that other animals ingest clay because it neutralizes toxins in their diets. Since elephants eat many plants that contain toxic compounds, their licking of clay is probably triggered by toxins too. Which one of the following, if true, most strengthens the argument? ['Elephants do not lick clay at times when their diet consists almost entirely of toxin-free foods.', 'Some of the clay that elephants lick contains minerals that are nutritionally essential for any animal.', 'The clay that elephants ingest does not contain any compounds that are toxic to them.', 'Elephants typically drink water immediately before or after they lick clay.']