# PTE Reading MCS #8: Scientific Theories on...

Reading: Multiple Choice, Single Answer #8 - Level 2 - Feeds Reading

> Read the text and answer the question by choosing one option. Only one answer is correct.

Theories are divided about the cause of the Permian mass extinctions. Some hypothesize that the impact of a massive asteroid caused a sudden disappearance of species. However, a look at the carbon- isotope record suggests that existing plant communities were struck down and re-found several times. To produce such a pattern would require a succession of asteroid strikes thousands of years apart. Other theorists have proposed that volcanic explosions raised the carbon dioxide level leading to intense global warming. One problem with this theory is that it cannot explain the massive marine extinction at the end of the Permian period. A new theory posits that rising concentrations of toxic hydrogen sulfide in the world's oceans plus gradual oxygen depletion in the surface waters caused the extinctions. Fortunately, this theory is testable. If true, oceanic sediments from the Permian period would yield chemical evidence of a rise In hydrogen sulfide-consuming bacteria.

The primary purpose of the passage is to___

- Present several hypotheses concerning the cause of the Permian mass extinctions.
- Discuss the strengths and weaknesses of the asteroid hypothesis of the Permian mass extinctions.
- Propose that theories regarding the cause of the Permian mass extinctions be tested.
- Argue that Permian mass extinctions could not have been caused by a volcanic explosion.

## Answer

Present several hypotheses concerning the cause of the Permian mass extinctions.

## Explanation

The passage is structured as a survey of different scientific viewpoints, starting with asteroid impacts, moving to volcanic activity, and concluding with a new theory about toxic oceans.

## Hints

- Look at how the passage introduces each new theory.
- Does the passage focus on proving one theory, or does it list several?

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