In Humans, Clear Gender Differentiation Occurs Not at Fertilization, but After

In humans, clear gender differentiation occurs not at fertilization, but after the second month of gestation. What is the first event of this differentiation?

A) formation of testosterone in male embryos
B) formation of estrogens in female embryos
C) anatomical differentiation of a penis in male embryos
D) activation of the SRY gene in male embryos and development of testes
E) activation of the SRY gene in females and development of the ovaries