What is the structure that holds together the two sister chromatids that form a chromosome?

I am pretty sure that it is called a Centromere. :)

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.

i don't understand

explanation:

i don't understand your question