The surface layer of the water freezes solid creating a barrier that insulates the ice below so that it keeps a steady temperature usually a few degrees above freezing (32 degrees).
Answer:
E. Variations in Earth's orbit occur gradually over 100,000 years.
In the coding region, natural selection tends to eliminate all of the mutations because of the high importance these regions have. The coding region contains genes that synthesize proteins and the changes in the DNA sequence can have devastating effects on the cell. Therefore, there are very few differences in the sequences of coding regions that can help us trace the lineage.
On the other hand, in the non-coding regions, the mutations often accumulate because they have little effect on the cell and the adaptive value of the organism. This enables us to trace up the lineage by comparing the sequences and seeing the differences in the sequences.
Introns because they are coding sequences.
Exons because they are non-coding sequences.
