In the following question, which one set of letters when sequentially placed at the gaps in the given letter series shall complete it?
PE_NP_ON_EO_
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r_pr_p_q_r_p
b_d_ed_b_c_e
gh_i_hi_ig_ii_ _
b_a_c_bc_b_ a
a _ c a a b _ a _ b c a a b _ a _ b _ a