With Quizz in CoursePro what is the ?try in URL?

Hello, in CoursePress Pro, I create a course with a quizz.
But when I (as a visitor) see this (free) course all works except the quizz. Choises are inactive, I can't select one unless in URL I supress "?try" at the end.
For exemple this url is like this :
http://mydomain.com/user123/courses/mynewcourse/modules/module-1-installation/page/2/?try

Then if I supress it and I load this URL without "?try" it works well I so I can answer and choose a answer.

Can you help me with this please ?

Thanks you in advance.