What determines the sex of a child?

I - The sex of the child is determined by the mother's genes.
II - The sex of the child is determined by the father's genes.
III - There is a 50% chance of having a boy or a girl.
a) Only I is correct.
b) Only II is correct.
c) Only III is correct.
d) II and III are correct.
e) I and III are correct.