Designing a software on quiz system

I am designing a quiz software, where i kept 2 button next button and previous buttton, in my software the questions are generated randomly so when user selects the and click the next button it goes to next question. Now the problem is when the user want review the previous question attemped and click the previous question the answer he selected and correct answer. My doubt is how to avoid the user to change answer in previous attempted and returning to same question where  he left. The thing as questions are selected randomly a new question appear in next button and the same he left.