Which of the following letter clusters will replace the question mark (?) and complete the following letter cluster series?

DW, FU, HS, JQ, LO, ?

NM

Explanation:

DW, FU, HS, JQ, LO, ?

The logic followed here is :

D → (+2) → F → (+2) → H → (+2) → J → (+2) → L → (+2)  → N → (+2) 

W → (-2) → U → (-2) → S → (-2) → Q → (-2) → O → (-2)  → M → (+2) 

So, the letter-cluster that replaces the (?) to complete the series is "NM"