Read the following statements and determine how many of them are correct:
$(1)$ Population is the total number of individuals of a species in a given area at a given time.
$(2)$ Developed countries show a low birth rate.
$(3)$ An equal proportion of reproductive and post-reproductive age individuals indicates a stable population.
$(4)$ In the presence of limited resources,the growth curve is sigmoid ($S$-shaped).

• A
  $2$
• B
  $3$
• C
  $1$
• D
  All statements are correct.