Select the combination of letters that when sequentially placed in the gaps of the given letter series will complete the series.
_ _ Q D P C _ _ Q C Q _ _ C P D Q C _ D P _ P D

this series consists of four alphabets QP and CD and each Q pairing with  C and D respectively and each P again pairing with C and D respectively making three pairs in total 

firstly if we see the given letters we notice that a similar set of letters are repeated in some kind of sequence 

it is also noticable that the after every Q and P there is C or D 

and after every set of two Q's there is  set of two P's  

and C and D are written alphabetically

so we can infer that the missing words are in following series is Q C Q D P C P D Q C Q D P C P D Q C Q D P C P D