listBox1.Items.Clear();
int num = Convert.ToInt32(textBox1.Text, 10);
if(checkBox1.Checked)
for (int i = 2; i <= num; i++)
{
bool prime = true;
for (int j = 2; j <= (int)i / 2 && prime == true; j++)
if (i %j == 0) prime = false;
if (prime) listBox1.Items.Add(i.ToString());
}
else
{
for(int...