Which of the following horizons has the most fertile soil? a- a horizon b-b horizon c-c horizon d- o horizon

A  A-horizon

Explanation:

The A Horizon is present below the O-horizon. The A-horizon is usually the top or first layer of the soil it is also called the topsoil. This soil zone is formed due to accumulation of organic matter and nutrients. It contains most of the organic matter. Thus, A-horizon has the most fertile layer of soil.

1) gene flow

- it is described as the transfer of genes from one population to another population so that frequency of gene is increased.

-a zebra migrates to join a different herd of zebras.

2) natural selection

- a mechanism of evolutionary change in which individuals with advantageous traits survive and reproduce whereas the others ( those that can not adapt) eliminate from the population.

-competition for sunlight leads to taller trees.

3) mutation

- permanent change in the nucleotide sequence of dna ( genetic material) that could have harmful or useful consequences.

-the dna of a snake changes to make its venom stronger.

4) genetic drift - it is a random phenomenon ( that occurs by chance such as fire) that results in change in the gene frequency in a small population.    

-a grassfire randomly sweeps through a population of buffalo and kills most of the animals.

explanation:

the backbone of the dna and rna compound is sugar phosphate.