गोविन्द भाष्य (श्रील बलदेव विद्याभूषण - अचिन्त्यभेदाभेद वेदान्त ब्रह्मसूत्र भाष्य सटीक)

saṁskāra-parāmarśāt tad-abhāvābhilāpāc ca
saṁskāra-of the purificatory rituals; parāmarśāt-because of the reference; tad-of
them; abhāva-of the non-existence; abhilāpāt-because of the explanation; ca-also.
This is also so because the scriptures state both the necessity of undergoing the
saṁskāras (rituals of purification) and the exclusion of the śūdras from these
rituals.
Purport by Śrīla Baladeva Vidyābhūṣaṇa
In the śruti-śāstra is the passage aṣṭa-varṣaṁ brāhmaṇaṁ upanīyata tam
adhyāpayed ekādeśe kṣatriyaṁ dvādaśe vaiśyam (One should perform the
saṁskāra and teach a brāhmaṇa boy when he is eight years old a kṣatriya boy when
he is eleven years old, and a vaiśya boy when he is twelve years old). This shows
that brāhmaṇas are eligible to study the Vedas because they are also eligible for the
saṁskāras. The scriptures also say nāgnir na yajño na kriyā na saṁskāro na vratāni
śūdrasya (A śudra is not allowed to light the sacred fire, perform a fire-sacrifice,
perform religious rituals, undergo the saṁskāras, or follow vows of penance). In
this way it is established that because a śūdra is not allowed to undergo the
saṁskāras he is also not allowed to study the Vedas.
Now he confirms the view that the śūdras are not eligible for the saṁskāras.
Sūtra 37
tad-abhāva-nirdhāraṇe ca pravṛtteḥ
tat-of that; abhāva-of the non-existence; nirdhāraṇe-in ascertaining; ca-also;
pravṛtteḥ-because of endeavor.
(This is so) also because care is taken to determine that (a student) is not (a
śūdra).
Purport by Śrīla Baladeva Vidyābhūṣaṇa