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« on: July 09, 2007, 03:44:53 PM »
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#### HtownsFinest

##### Re: Test 18 / Inference Question
« Reply #1 on: July 09, 2007, 05:19:22 PM »
A. I will explain later if I decide to forgive you for your obnoxious parody thread.

#### HtownsFinest

##### Re: Test 18 / Inference Question
« Reply #2 on: July 09, 2007, 07:32:37 PM »
Dec 92 . Section 2 . Question 20.

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.

Question: 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 cloud's 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-18 than it retains.

Will someone go through this answer by answer and give the rational for each one as to why it's correct/incorrect? Thanks.

A. Correct. It's pretty much explicitly stated in the text: "In a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen." Once it is formed... it is in existence as a rain cloud; thus the aforementioned evidence must apply.

B. If the cloud is dropping a higher concentration of O-18 H20 molecules during rain fall than the cloud's concentration, you cannot infer that the cloud will contain a greater-than-normal percentage of O-18.

C. "But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth."  Higher than the proportion in a typical cloud. Does not infer that O-18 concentration advances beyond regular O concentration.

d. Completely baseless.

e. If it is surrendering O-18, but ends up with the same concentration in the cloud after the rainfall, it does not necessarily surrender more than it retains.