19.2.4 (Weaken Questions Problem Set 13)

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I don't see how D seriously undermines the hypothesis. How do we know anything about the death of diatoms? For this answer to work, you have to assume that Antarctic diatoms die near Antarctica (why can't they move or float away, or the death shells float away?) Lastly, don't you have to assume that the sediment left by the death shells would be indicative of a population increase? Aren't life and death two totally different ideas?. How are we supposed to know that these are OK assumptions?

Take for instance A (just for the sake of argument, I understand that A is incorrect). I think A would work if you assume diatoms of today are similar to diatoms during the ice age. You would also need to assume that the "unusually large amounts of ferrous material" that does not exist today would not promote a further increase in their population today. How are these assumptions less reasonable than the ones needed for D to be correct?