while selection of main chain if 2 or more chains of equal lengths are possible then which will be te main chain?

e.g. C(Cl₃)-C[C(NO)₃]-C-C[p C₆H₅Cl]-C-C(C₆H₅)

'x' gm of a compound A3B2C5 contains 'y' gm of A atoms

 

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how to represent the possible fischer projections for 2-bromohexane