Answer:
C. Characters with little genetic variation will constrain the rate of natural selection
Explanation:
Natural selection can have an effect only as long as there are variations on characters on which it can select upon. Genetic variation is independent of the environment or the needs that a species may have to become better adapted to its changing environment. In other words, genetic mutations are the raw material on which natural selection can work. If there is a low rate for mutations to occur, then natural selection will proceed at a slow pace. The opposite is also true. Since mutation rate may be more frequent at some locations in a gene than at others, it can be predicted that the characters coded in that gene location will be more likely to be affected by natural selection and therefore what phenotype may be molded by natural selection.
Answer:
The answer should be 
Since the pedigree has two different letters, it will have one letter of each, but the DNA will still be A.
C. because stimuli is how your body responds to something, and in this case, someone's heart rate increases WHEN their blood oxygen drop. Do you see how it responds? ;-)
