The science that describes populations is called gerontology psychology demography geography

We can identify which of the scientific studies describe population thru their definitions.

Gerontology is particular with old age, its process and the problems are encountered by old people.

Psychology studies the feeling, behavior, and mind of people, including all phenomena.

Demography shows the structure of human population, how it changes, based on the statistics of births, deaths, income, or occurrence of disease, the composition of a particular human population.

Geography is understanding of the Earth.  And it's complexities.

Correct answer then is demography.

a. as the temperature increases, the kinetic energy of the gas molecules increases & the molecules move faster & further away from each other.

b. if you decrease the temperature of a fixed amount of gas at constant pressure, the combined gas law reduces to charles's law. in this case, volume will also decrease.