How to close current form in C# and open the second form as modal dial

Question

I have a scenario in c# where I open first form dialog log call it form one. Form one has a button which should open Form two. My issue is upon clicking button in form one I want to close form one and show form two as modal dialog and form two shows and still form one is not closed. Here is sample code I have tried which is not working

private void button1_Click(object sender, EventArgs e)
{
    Form frm=null;
    if (radioone.Checked == true)
    {
        frm=new frmPayByCash(this);
    }
    else if (radiotwo.Checked == true)
    {
        frm = new frmTwo(this);
    }
    else
    {
        frm = new frmOne(this);
    }
    if (frm != null)
    {
        this.Hide();
        frm.Closed += (s, args) => this.Close(); //have tried this but its closing both forms at ago
        frm.Show();
    }
}