What type of system is required to maintain the amount of sugar in the blood at a certain level, making sure it does not rise too high or fall too low?

The Digestive system.

Explanation:

Even though the Cardiovascular system pumps the blood in the body, blood sugar is dependent on the regulating hormones insulin and ketone. Which are created in the liver. And the liver is part of the Digestive system. Diabetes, hypoglycemia and hyperglycemia are results of a faulty liver.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."

an organism's phenotype is the  form of a trait  that it  displays. for  flowercolor, a pea  plant  can  display  a phenotype of purple or white flowers. an organism's  genotype  is the alleles of a gene it contains. based on his data, mendel concluded that a phenotype can be  determined  by more than one  genotype.