Sedimentary rock hardens within the earth' s crust as layers of matter accumulate and the pressure of the layers above converts the layers below into rock. One particular layer of sedimentary rock that contains an unusual amount of the element iridium has been presented as support for a theory that a meteorite collided with the earth some sixty million years ago. Meteorites are rich in iridium compared to the earth' s crust, and geologists theorize that a meteorite' s collision with the earth raised a huge cloud of iridium-laden dust. The dust, they say, eventually settled to earth where it combined with other matter, and as new layers accumulated above it, it formed a layer of iridium-rich rock. Which one of the following, if true, would counter the claim that the iridium-rich layer described in the passage is evidence for the meteorite collision theory? ["The huge dust cloud described in the passage would have blocked the transmission of sunlight and lowered the earth's temperature.", 'Layers of sedimentary rock are used to determine the dates of prehistoric events whether or not they contain iridium.', 'A layer of sedimentary rock takes millions of years to harden.', 'Sixty million years ago there was a surge in volcanic activity in which the matter spewed from the volcanoes formed huge iridium-rich dust clouds.']