Select the letter-cluster that can replace the question mark (?) in the following series.
PROGRESS, QTOGRESS, QTRKRESS, ? , QTRKWKZA
Here in every step of the series two consecutive letter of the first word is changed, like in first part, "PR" in PROGRESS is changed in a certain way like the alphabets are changed in a sequence of +1, +2, +3, +4, +5, +6, .... etc.
So, after, "QTRKRESS", R and E is replaced by R+5 = W and E+6 = K respectively to the missing word in the series is 'QTRKWKSS'.
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