Question

walter alvarez, a professor at the university of california, berkeley, is best known for proposing the theory that the dinosaurs were killed off by an asteroid strike 66 million years ago. beyond the immediate effects of fire, flood, and storm, dust darkened the atmosphere, cutting off plant life. many animal species disappeared as the food chain was snapped at its base. alvarezs main evidence is an abundance of iridium in the kt boundary, a thin stratum dividing cretaceous rocks from rocks of the tertiary period. iridium normally accompanies the slow fall of interplanetary debris, but the kt boundary strata implies ________
which choice most logically completes the text?
○a) a rapid, massive deposition, given its iridium content is 10 - 100 times more abundant than normal.
○b) whole species of small organisms vanishing from the fossil record.
○c) the formation of basaltic spheres and deformed quartz grains, both of which could have resulted from high - velocity impact.
○d) that iridium in the kt boundary was gradually deposited over a period of 50,000 to 100,000 years.

Explanation:

The text mentions that iridium normally comes with the slow fall of inter - planetary debris. But the key here is the contrast. The KT boundary has an abundance of iridium. If it were a slow process (like the normal inter - planetary debris fall), the iridium content would be low. But since it's 10 - 100 times more abundant (as in option A), it implies a non - slow (rapid and massive) deposition. Option B is about species vanishing which is not directly related to the iridium - deposition link. Option C is about other geological formations (basaltic spheres and deformed quartz grains) not related to the iridium - based argument. Option D suggests a gradual (slow) deposition which goes against the "abundance" (since slow would mean less accumulation in a given stratum) and the contrast with the normal slow inter - planetary debris fall.

Answer:

A. a rapid, massive deposition, given its iridium content is 10 - 100 times more abundant than normal.