Can't create code that selects 10 out of 25 different strings uniquely C#

I am trying to make a simple math program in C# I have created a button, textbox and set 25 variable to strings (sentences, in this case the questions). I want the program to pick 10 of the questions with no duplicates when i hit the button and display the strings in the textbox.

Main problem is cant get the program to pick 10 unique strings, appreciate your help