Whats wrong in this JS

Question

https://code.sololearn.com/W9txrnI28G76/?ref=app

Answer 1

just include jquery in head tag.
https://code.sololearn.com/WlT6VNYTestJ/?ref=app
https://code.sololearn.com/WV75aekl928L/?ref=app

Answer 2

<script src="https://code.jquery.com/jquery-3.2.1.js"></script>
where is your jQuery CDN.