The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
8. And the best (i.e. the chief vital air).
The Sûtra extends to the chief vital air (mukhya prâna) a quality already asserted of the other prânas, viz. being an effect of Brahman.--But, an objection may be raised, it has already been stated of all prânas without difference that they are effects of Brahman; e.g. the passage, 'From him
is born breath, mind, and all organs of sense' (Mu. Up. II. 1, 3), states the origin of prâna separately from the senses and the manas; and there are other passages also such as 'He sent forth prâna' (Pr. Up. VI, 4). Why then the formal extension?--We reply: For the purpose of removing further doubt. For in the Nâsadiya-sûkta whose subject is Brahman there occurs the following mantra: 'There was neither death nor the Immortal; nor manifestation of either night or day. By its own law the One was breathing without wind; there was nothing different from that or higher than it' (Ri. Samh. X, 129, 2). Here the words, 'was breathing' which denote the proper function of breath, intimate that breath existed as it were before the creation. And therefrom it might be concluded that prâna is not produced; an idea which the Sûtrakâra discards by the formal extension (to prâna of the quality of having originated from Brahman).--Moreover the word 'breathed' does not intimate that prâna existed before the creation; for in the first place it is qualified by the addition 'without wind,' and in the second place scriptural passages--such as 'He is without breath, without mind, pure' (Mu. Up. II, 1, 2)--declare expressly that the causal substance is without any qualifications such as prâna and so on. Hence the word 'breathed' has merely the purpose of setting forth the existence of the cause.--The term 'the best' (employed in the Sûtra) denotes the chief vital air, according to the declaration of scripture, 'Breath indeed is the oldest and the best' (Kh. Up. V, 1, 1). The breath is the oldest because it begins its function from the moment when the child is conceived; the senses of hearing, &c., on the other hand, begin to act only when their special seats, viz. the cars, &c., are formed, and they are thus not 'the oldest.' The designation 'the best' belongs to the prâna on account of its superior qualities and on account of the passage, 'We shall not be able to live without thee' (Bri. Up. VI, 1, 13).