If a person has a blood-calcium (Ca) level of 8 mg/100 mL of blood, which of the following mechanisms does the body use to maintain blood-calcium homeostasis?
A The kidneys take up more Ca? and release vitamin D.
B The bones release Ca?
C The intestines increase reabsorption of Ca?
D All of the above

The kidneys take of up more Ca2+ and release vitamin D. The bones release Ca2+. The intestines increase reabsorption of Ca2+.

b) In the population, there must be one or more traits for which individuals differ.

Explanation:

Natural selection refers to the differential process of survival and reproduction caused by phenotypic differences among individuals in particular environmental conditions. These phenotypic differences are due to genetic variation among individuals, which is the primary source on which the natural selection act. In consequence, natural selection is composed of three main components: 1-variation among individuals, 2-inheritance of the phenotypic traits that vary within a population or species, and 3- different survival and reproduction.