The passage suggests that which of the following is most likely to have been true of the oxygen-16 and oxygen-18 isotopes found in the lower sections of the GRIP core? A. There was significantly more isotope oxygen-18 than isotope oxygen-16 in the ice layers. B. There was significantly more isotope oxygen-16 than isotope oxygen-18 in the ice layers. C. Ratios of isotopes oxygen-18 and oxygen-16 varied in the ice layers. D. Layers containing isotope oxygen-18 placed stress on the layers containing isotopeoxygen-16, possibly distorting them. E. Isotope oxygen-16, being lighter, was located mainly in the upper layers, whereas oxygen-18 had settled into the lower layers.