Create the variable priceList, that stores the list element.

<html>
<body>
<h2>Museum Tickets</h2>
<ul id="prices">
<li>Adult</li>
<li>Child</li>
<li>Student</li>
</ul>
<script>
var priceList;
</script>
</body>
</html>]]>

Make sure to type var priceList.