How do you think scientists count the world’s human population?

Scientist count the world humans population with an organization called The United Nations Population Division (UNPD),

Explanation:

Which keeps track of the world population, projects that the world's human population will hit 7 billion on Halloween Day 2011. Admittedly, that is just an estimate: There's no way to know exactly how many people are alive at any given moment, and the true date that humanity's ranks will surpass 7 billion could come in the weeks or months before or after Oct. 31. Nonetheless, the UN's guess is the best there is.

i think it's c