PrepTest 117, Section 4, Question 17

What we’re looking for

Working the answers

• A

  This supports the timing. The regular sequence of ice ages described here began around 800,000 years ago. If Earth wasn't passing through cosmic dust before then, the absence of that proposed cause lines up with the absence of this sequence. That doesn't prove causation, but it makes the suggested connection more plausible.

• B

  This supplies both the timing and the dense dust. A collision 800,000 years ago could explain why the material needed for the hypothesis became available when this sequence of ice ages began. The dust's continued orbit also leaves it available for later encounters. That's useful support, even though B doesn't show that Earth passed through it on every relevant occasion.

• C

  Don't dismiss this just because the dust comes from volcanoes. This answer supports the proposed cooling mechanism: putting dust into Earth's atmosphere is followed by lower temperatures. A slight drop isn't an ice age, and volcanic dust isn't cosmic dust, but the observation still gives us a reason to take the dust-causes-cooling step seriously.

• D
  Correct

  Rocks hitting Earth and kicking up dust from its surface are a different explanation. The hypothesis is that Earth passes through dense clouds of cosmic dust, which then enters the atmosphere. D doesn't provide evidence of those clouds or those encounters.

  The word cosmic makes this sound relevant, but follow the actual sequence: a rock arrives, then dust rises from Earth. C offered evidence that atmospheric dust can cool the planet. D merely offers another way dust might get into the atmosphere; it doesn't even report a cooling effect.

• E

  This is the kind of physical evidence we were looking for. Material associated with cosmic debris appears in sediment from the same times as the ice ages. That ties the proposed cause to the periods we're trying to explain. It doesn't establish every step, but it strengthens the hypothesis.