The human embryo contains all the genetic instructions necessary to become either male or female. The male embryo contains a master regulatory gene that acts as a biological switch,turning other genes on and off. The absence of this gene results in a female,which remains sexually immature. This master regulatory gene is located on the .....

• A
  Homologous part of the $X$-chromosome
• B
  Non-homologous part of the $X$-chromosome
• C
  All autosomes
• D
  $Y$-chromosome