The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
49. On account of there being injunction of the others also, in the same way as of the state of a Muni.
As the state of the Muni (Samnyâsin) and the state of the householder are enjoined in scripture, so also the two other orders, viz. that of the hermit, and that of the student. For we have already pointed above to passages such as Austerity is the second, and to dwell as a student in the house of a teacher is the third.' As thus the four âsramas are equally taught by scripture, they are to be gone through equally, either in the way of option (between them) or in the way of comprehension (of all of them).--That the Sûtra uses a plural form (of 'the others') when speaking of two orders only, is due to its having regard either to the different sub-classes of those two, or to their different duties.