I believe part A is simple, or I'm just full of it:
A) The "current" (at least for this statement) population is claimed to be 6 billion.
Now the contradiction is, it can't keep growing at 1.3%. The next year it will grow by 1.3% of 6 billion, but after that, 6 billion will have changed. So the following year, it's not gonna grow by 1.3% of that new value.
As for the other, it's the same idea. The 1.3% applies to the 6 billion, as does the 78 million. So the next year, 6 billion will have changed, and therefore 78 million cannot be the following year's increase.
I don't know if you follow me on that. As for the other two parts, I have no clue.