After several miles of running and sprinting in intervals, the runners muscles ache and feel fatigued. Which is the best explanation for what has occurred.

The runner's muscles have accumulated lactic acid.

what's the rest of the question

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.