Identify the correct statements regarding the use of stable oxygen isotopes in reconstructing ancient climates.

a. Oceanic microorganisms preserved in sea floor sediments or elsewhere can be analyzed to measure the ratio of 16O to 18O.
b. As global temperature varies, the amount of 18O in water changes in response.
c. Oxygen is incorporated into biological tissues of living organisms.
d. The ratio of 14C to 18O provides an estimate of global temperature.