The four bases in DNA are: adenine, thymine, cytosine, guanine.
In RNA, thymine gets replaced by uracil. 
Purine is only a name for the two bases guanine and adenine. 
        
             
        
        
        
Limestone deposits can help researchers learn about what the area was like  thousands of years ago as Limestone can contain fossilized plants and animals.
Explanation:
- Limestone often contains fossils of shelled sea creatures. Entire reef formations and communities of organisms are found preserved in limestone.
- The types of fossils found in limestone include coral, algae, clams, brachiopods, bryozoa and crinoids.
- Limestone is a sedimentary rock made almost entirely of fossils. 
- Fossils are the remains of ancient plants and animals, like an imprint in a rock or actual bones and shells that have turned into rock. Fossils are found in sedimentary rocks and hold the clues to life on Earth long ago.
- Limestone is composed of the mineral calcite; calcium carbonate. It often has variable amounts of silica in it, as well as varying amounts of clay, silt, and sand. Limestone rocks fall under the category of sedimentary rocks that are made from mineral calcite.
 
        
             
        
        
        
Answer:
Each component is in balance with the other components. As long as the components are in balance, the ecosystem can remain stable and healthy. Ecosystems may remain stable for many years if the different components are balanced.
 
        
                    
             
        
        
        
Answer: The correct answer would be gene pool.
Explanation:
Gene pool refers to the sum of all the alleles in an individual.
The allele refers to the alternate forms of the gene.
Large population usually has more genetic diversity as compared to small population. Variation in gene pool is essential for survival of a population.