The collection of questions increases with 10 questions, every time I resolve the quiz

I am writing a C# program to display a test of questions.
The test has 10 questions.
But the program doesn't want to follow the same algorithm if I select a new quiz with the help of

   private void newToolStripMenuItem_Click (object sender, EventArgs e) 
 

The quiz doesn't stop after 10 questions.It goes on, it repeats the questions and at a certain number of questions it blocks.

I stepped through the code,and I saw: when I resolve the quiz for the first time:questions.Count=10; when I resolve it for the second time: questions.Count=20; when I resolve it for the third time: questions.Count=30; Would you like to tell me how can I have questions.Count=10 for every quiz, please ?

Here is my code:

      public partial class Form3 : Form
 {
     int nr;
     Collection<question> questions = new Collection<question>();
 
     public class question
     {
       public bool displayed = false;
       public string text;
      }
 
 
     //when I press a button from MenuStrip my quiz begin
     private void newToolStripMenuItem_Click(object sender, EventArgs e)
     {   nr = 1;
         button1.Enabled = true;//it's the next_question button
 
         StreamReader sr = new StreamReader("quiz.txt");
         while (!sr.EndOfStream)
         {   
             question i = new question();
             questions.Add(i);
         }
         sr.Close();
         int x = r.Next(questions.Count);
         textBox1.Text = questionText;
         questions[x].displayed = true;
         current_question=x;
 
       }
 
 
       private void button1_Click_1(object sender, EventArgs e)//it's the code for the                                                                                                
                                                                  next_question button
       {   
        if (nr >= questions.Count)
                              {
                               button1.Enabled = false;
                               }
         else
     {
         int x;
         do   { x = r.Next(questions.Count); } 
               while (questions[x].displayed == true);
         textBox1.Text = questionText;
         questions[x].displayed= true;
         nr++;
         current_question = x;
    }
 

}

I add that I tried to create

       Collection<question> questions = new Collection<question>()
 

for every quiz I resolve, putting this at the beginning of the

       private void newToolStripMenuItem_Click(object sender, EventArgs e) 
 

or at the end of my current quiz:

       if (nr >= questions.Count){ //here } 
 

None of these changes prevent the collection to increase with 10 questions.
Thank you!

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