According to the passage, which of the following is true of the ratios of oxygen isotopes in ocean sediments?
A.They indicate that sediments found during an ice age contain more calcium carbonate than sediments formed at other times.
B.They are less reliable than the evidence from rocks on land in determining the volume of land icE.
C.They can be used to deduce the relative volume of land ice that was present when the sediment was laid down.
D.They are more unpredictable during an ice age than in other climatic conditions.
E.They can be used to determine atmospheric conditions at various times in the past.