4. 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.

5. It can be inferred from the passage that precipitation formed from evaporated ocean water has

（A） the same isotopic ratio as ocean water

（B） less oxygen 18 than does ocean water

（C） less oxygen 18 than has the ice contained in continental ice sheets

（D） a different isotopic composition than has precipitation formed from water on land

（E） more oxygen 16 than has precipitation formed from fresh water

6. According to the passage， which of the following is （are） true of the ice ages？

I. The last ice age occurred about 25，000 years ago.

II. Ice ages have lasted about 10，000 years for at least the last several hundred thousand years.

III. Ice ages have occurred about every 100，000 years for at least the last several hundred thousand years.

（A） I only

（B） II only

（C） III only

（D） I and only

（E） I， II and III