In humans, when do the secondary sexual characteristics first appear?

1. dermis

2. chemical

3. urine

4. viruses

5. antibodies

i think the answer is b.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.