procedure TForm1.bErzeugeClick(Sender: TObject);

  function RTerm : string;
  var
    s : string;
  begin
    s := 'AB';
    if random < 0.3 then s := s+RTerm;
    s := s+'C';
    if random < 0.3 then s := s +RTerm;
    result := s;
  end;

begin
  mAusgabe.Lines.Add(RTerm);
end;