Look at the dna sequence GAA-TTC-GCA what do the g and a repersent in the dna sequences

Why are scientists concerned about the depletion of the ozone levels in the stratosphere?

Which of the following factors would not contribute to allopatric speciation? a) a population becomes geographically isolated from the parent population.b) the separated population is small, and genetic drift occurs.c) the isolated population is exposed to different selection pressures than the ancestral population.d) different mutations begin to distinguish the gene pools of the separated populations.e) gene flow between the two populations is extensive.

What is the difference between topsoil and clay?