What If Mars Were Inhabitable by Humans and We Had

Multiple Choice

What if Mars were inhabitable by humans and we had developed the technology to transport and settle half the world's population there? At present birth rates, how long would it be before we were faced with not one but two world populations of 6 billion? (Hint: In what year did the world population hit 3 billion?)

A) millions of years
B) thousands of years
C) hundreds of years
D) tens of years