PT18. S2. Q19-20

The 180 Bro_OVO

Literally, No idea.


Oxygen.18 is a heavier-than-normal isotope of oxygen. In a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen. But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth. Consequently, scientists were surprised when measurements along the entire route of rain clouds' passage from above the Atlantic Ocean, the site of their original formation, across the Amazon forests, where it rains almost daily, showed that the oxygen-18 content of each of the clouds remained fairly constant.

19. Which one of the following statements, if true, best helps to resolve the conflict between scientists' expectations, based on the known behavior of oxygen-18, and the result of their measurements of the rain clouds' oxygen-IS content?

(A) Rain clouds above tropical forests are poorer in oxygen-18 than rain clouds above unforested regions.

(B) Like the oceans, tropical rain forests can create or replenish rain clouds in the atmosphere above them.

(C) The amount of rainfall over the Amazon rain forests is exactly the same as the amount of rain originally collected in the clouds formed above the Atlantic Ocean.

(D) The amount of rain recycled back into the atmosphere from the leaves of forest vegetation is exactly the same as the amount of ram in river runoffs that is not recycled into the atmosphere.

(E) Oxygen-18 is not a good indicator of the effect of tropical rain forests on the atmosphere above them.

20. Which one of the following inferences about an individual rain cloud is supported by the passage?

(A) Once it is formed Over the Atlantic, the rain cloud contains more ordinary oxygen than oxygen-18.

(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18.

(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen.

(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18.

(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains.

c.janson35

For 19, it is essential that you have a strong grasp on why the scientists were surprised at their findings that the amount of o18 was the same throughout the clouds' routes.

We know that there is less o18 than normal o, and that when it rains, a higher proportion of all o18 falls than all ordinary o. This would seem to indicate that, as it rains, there would be a consistently lower amount of o18 in clouds because a lot of it is falling. But how is it the case that, when it rains a lot over rainforests, the amount of o18 is still the same as the beginning? We need some sort of information that would help account for this surprising lack of deficit. Something that provides us with more o18 over the forests that would make up for what is lost.

A: if anything, make the fiding more surprising. It would seem that o18 would be even less over forests, not same.

B: hmm, this seems like it maybe accounts for why there isn't a difference. The forests can replenish the clouds as it rains. Maybe this is how the o18 doesn't diminish.

C: again, if the amount of rainfall is the same as what was collected originally, we would expect there to be less after it rains. But there is the same o18 so this answer doesn't add anything.

river runoff? No idea where this is coming from and don't know why it has any bearing of our comparison between clouds over the ocean and clouds over the forests.

E: o18 is not a good indicator? Our whole conclusion is about why there isn't o18 so we certainly don't want an answer that says o18 is untrustworthy. In any case, this answer does nothing to make up for the lack of deficit.

B, then, is the only answer that would show us how the o18 levels are not decreasing. Because they can be made up over the forests.

Hope this helps!

c.janson35

For number 20, you can answer this question by just reading the first sentence. In rain clouds, molecules containing o18 are rarer than molecules containing ordinary o.

This is almost exactly what A says. If o18 molecules are rarer than normal o molecules, then it is very strongly supported that there is more normal o than o18.

The 180 Bro_OVO

@c.janson35

That was PERFECT. Thank you!