Answer:
b. the rate at which the mRNA is degraded.
Explanation:
DNA is transcribed into mRNA which then joins ribosomes present in the cytoplasm to serve as a template for protein synthesis. Regulation of gene expression occurs at several levels. 
One such regulatory mechanism is the stability of mRNA and the rate of its degradation which in turn affects the availability of template for the process of protein synthesis and thereby keeps a check on the total amount of protein being formed in a cell.
For example, a very short life span of bacterial mRNA (few minutes) allows them to regulate the pattern of protein synthesis quickly in response to the changes in surroundings. 
 
        
             
        
        
        
Answer:
At the bottom of the sequence
Explanation:
According to the "principle of superposition of strata", the oldest rocks are formed at the bottom of sedimentary rock sequences. 
The oldest rock layer would contain the the oldest fossil. Weathering, erosion, transportation and deposition would bring all sediments which includes rock materials and remains of living organisms to the basin where they would be lithified to form sedimentary rocks. This makes the oldest fossil to be at the bottom of the sequence. As time progresses, the fossil sequence would continue to accumulate and young upwards as the strata becomes relatively younger. Also, we know that fossils succeed one another in a definite manner. 
This why relative dating of rocks can be possible. 
 
        
             
        
        
        
A model of a globe is a physical model. The limits are that the solar system is too big to be seen all at once. From the Tennessee science textbook, "Many years ago, scientists thought that Earth was the center of the universe and the sky was a blanket that covered the planet. Later through the observation, it was discovered that the objects you see in the sky are the sun, the moon ,stars, and other planets. This new model explained the solar system differently.