Oxygen-18 and oxygen-16 are useful in studying past climates because

a. ozone formation and decay depend on the ratio of oxygen-18 to oxygen-16
b. both water and calcite contain oxygen
c. oxygen ratios are directly related to carbon dioxide (co2) levels
d. more oxygen-18 is stored in glacial ice when temperatures drop

B. both water and calcite contain oxygen.

Explanation:

water molecules are not equal some are substituted with a heavier counterpart,the standard Oxygen molecule can be substituted by the 18O and hydrogen by H2 given rise to H216O,HD16O and H118O they are termed isotopologues and gave different physical properties.First, over the ocean, the heavier isotopes are less likely to take part in the formation of moisture, leading to lower concentrations of the heavy isotopes in the clouds compared to the mean oceanic water isotopic composition (lower concentration means that the δ18O is more negative). Then, as the air masses move toward the poles, temperature decreases leading to precipitation (either liquid or solid). The heavier isotopes will be preferentially found in the condensed phase than the light ones, which depletes even more the cloud from its heavy isotopes. Finally, in remote areas of the Polar Regions, the isotopic content of the final precipitation results from successive precipitation events. Since, as we saw, the precipitation contents in each isotope are different, each successive precipitation event will have a different isotopic composition, with the final one having the least heavy isotopes – a process called distillation. At each step of the moisture’s path from ocean to cloud to precipitation, the isotopic fractionation is strongly influenced by temperature. This leads to a temperature signal in the isotopic composition of both vapour and precipitation

fertilization is the union of an egg and a sperm cell i believe.

c. seafloor spreading occurs.

d. crust becomes molten rock.

e. denser plates slide under lighter plates.