Oxygen isotope ratio and Glaciers The question is basically: 


  During the Cretaceous there were no continental    ice    sheets. The ratio
  of O-18/O-16 in sea water was:
  
  a) Higher than today
  b) Lower than today


The answer is stated to be b, but it does not make sense as glaciers have a lower 18O/16O ratio and so the ratio should be higher than today. Can someone justify why it's b?
It does make sense.
As you said - glaciers have lower 18O/16O. This is because they have less 18O overall. Therefore, the oceans have more 18O. But this is today.

At the Cretaceous there were no continental ice sheets, so what we said previously is not relevant. There are no ice sheets to take all of the 16O, so it's all in the ocean. This results in a lower overall 18O/16O ratio.