[A]: We can distinguish sexuality of an organism by hormone system.
[B]: Sex determination can be done by chromosomes.
Both A and B are correct but B is not the correct explanation of A.
Sex determination is influenced by hormones such as testosterone, estrogen, progesterone. The sex-determination system (XX/XY) is found in humans. In this system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY).