inserting value by combobox

SqlConnection conn = new SqlConnection();
           conn.ConnectionString = @"Data Source=.\SQLEXPRESS;AttachDbFilename=E:\project\Projects\secquestion and password change with new tarika insert\labelnameinsecondformtry\Database1.mdf;Integrated Security=True;User Instance=True";
           SqlCommand cmd = new SqlCommand();
           cmd.Connection = conn;
           conn.Open();

           try
           {
               cmd.CommandText = "INSERT INTO Table2 (username,password,sec_questions) VALUES (@username,@password,@sec_question)";
               cmd.Parameters.Add("@username", SqlDbType.VarChar).Value =textBox1.Text;
               cmd.Parameters.Add("@password", SqlDbType.VarChar).Value = textBox2.Text;
               cmd.Parameters.Add("@sec_questions", SqlDbType.VarChar).Value =comboBox1.Text;
               cmd.ExecuteNonQuery();//error is coming which is "Must declare the scalar variable @sec_question."
           }
           finally
           {
               conn.Close();
               MessageBox.Show("hello welcome");
           }