Which one set of letters when sequentially placed at the

Which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?
h_t_l_tt_lht_m_htt_l

The pattern of given letter series is :-
h t t m l
Using the same pattern, we will get complete letter series.
h t t m l/ h tt m l/ ht t m l /htt m l