( ( The pupil ) ( ( is ( ( the opening ) ( ( through which ) ( light ( enters ( the eye ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN pupil)) (VP (VBZ is) (NP (NP (DT the) (NN opening)) (SBAR (WHPP (IN through) (WHNP (WDT which))) (S (NP (NN light)) (VP (VBZ enters) (NP (DT the) (NN eye))))))) (. .)))
The pupil is the opening through which light enters the eye.
(ROOT (S (NP (NP (DT The) (NN pupil)) (PP (IN of) (NP (DT the) (NN eye)))) (VP (VBZ allows) (ADJP (JJ light) (S (VP (TO to) (VP (VB enter)))))) (. .)))
The pupil of the eye allows light to enter.