C# error please rectify

Question

s = txt.Text.ToUpper();

n=s.Length;

for (int i = 0; i <n; i++)

{

s = s[i].ToString();

lbl.Text = s;

Response.Write(lbl.Text);

Response.Write("\n");

txt.text="answer"(I am giving)

 

i am getting index was outside the bounds of the array