the extent to which it succeeds 
(ROOT (NP (NP (DT the) (NN extent)) (SBAR (WHPP (IN to) (WHNP (WDT which))) (S (NP (PRP it)) (VP (VBZ succeeds))))))
(ROOT (NP (NP (DT ) (NN )) (SBAR (WHPP (IN ) (WHNP (WDT ))) (S (NP (PRP )) (VP (VBZ ))))))