Which letter-cluster will replace the question mark (?) in the following series?

SQO, LJH, ECA, ?

XVT

Explanation:

Explanation:

SQO, LJH, ECA, ?

The logic followed here is :

S → (-7) → L → (-7) → E → (-7) → X 

Q → (-7) → J → (-7) → C → (-7) → V 

O → (-7) → H → (-7) → A → (-7) → T

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