What is the main concern about the world's population growth?

I- The increase in the number of people who will inhabit the Earth in the future.
II- The low birth rate in some countries.
III- The need for responsible parenthood.
IV- The increase in the number of people who are not in working age.
a) Only I and II are correct.
b) Only II and III are correct.
c) Only III and IV are correct.
d) Only I, II and IV are correct.
e) I, II, III and IV are correct.