When Organisms Living in Seawater Grow Shells,the Ratio Between the Isotopes

When organisms living in seawater grow shells,the ratio between the isotopes of oxygen (18O/16O) trapped in their shell is dependent on the sea temperature.Why might geologists collect these sea shells from deep-sea sediments that are Pleistocene in age and measure their oxygen isotope ratios?

A) They might want to know how cold the water was at the seafloor in the past.
B) They want to know how metabolism of the organism affects the oxygen isotope ratios.
C) The organisms live in the photic zone,and so photosynthesis must be the main factor in the oxygen content.
D) The organisms live near the surface,so their oxygen isotope ratios record surface temperature during Earth's great climate fluctuations of the ice age.