How to hide key answers in pdf file temporarily and then show upon request (classroom usage)

I have a pdf file has answer keys that I don't want the student to see before solving.http://www2.0zz0.com/2017/11/21/13/555028413.jpg " rel="nofollow noreferrer" style="font-style: inherit; font-weight: inherit; font-size: inherit; font-family: inherit; color: #50aec6;">https://www.0zz0.com][/url]

How to show the key answers upon my request _ after doing the practice with students orally.