Although the Colorado River has obviously cut down through the layers of rock, why were scientists unsure that this explained how some areas of the Grand Canyon formed? Please answer and I will mark you

uh two lol

"do a testcross with a homozygous recessive plant."

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.