The figure above shows the historic composition, as percentages, of

Question

https://gmatclub.com/forum/download/file.php?mode=view&id=60901 The figure above shows the historic composition, as percentages, of gases within the Earth's atmosphere. All gases depicted refer to their molecular forms, not to atoms of those elements within other compounds. (For example, oxygen refers to O_{2} ) https://gmatclub.com/forum/download/file.php?mode=view&id=60902 The table above shows the start dates for various time periods in Earth's geologic history. 1. A certain class of archean bacteria are unable to survive at the surface of the ocean if the concentration of molecular oxygen in the atmosphere is more than 10%. Of the eras for which data is known, in which era would these bacteria have first been unable to survive at the ocean’s surface? GRAPH_1_PLACEHOLDER 2. Based on the information presented in the first figure, it is GRAPH_2_PLACEHOLDER that the mass of nitrogen in the atmosphere began to decrease around 2.5 billion years ago. The Last question based on this stem can be found here. 10.jpg 11.jpg

Mona2019 · Answer

1. B
2. True 
3. C

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Sajjad1994 · Answer

OA of this question has been posted, it is:

B, C and B

Thank you

Mona2019 · Answer

Answer to qn 2 is C /uncertain because , the nitrogen level starts to decrease 2.4 billion years ago..but remains constant from 2 billion years and ahead.

Qn 3 answer is 2.4 billion years , because though carbon dioxide is present 4 billion years ago, atmospheric oxygen is found to be present 2.4 billion years ago. As photosynthesis uses carbon dioxide to produce oxygen, so answer would be 2.4 billion years ago.

sajjad  , could you please help with the OE for qn 2 & 3, incase you have it

Not sure if my analysis to get to the answers is correct.

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Sajjad1994 · Answer

Official Explanation

For the first question,  you first need to understand what you're being told to look for. Because you're looking for the first point at which the bacteria couldn't survive, you're looking for the point at which oxygen makes up more than 10% of the atmosphere. Notice that while oxygen first starts to be a noticeable part of the atmosphere around 2.4 billion years ago, that it's between 2.4 and 2 billion years ago that it is a big enough part of the atmosphere to cause major issues for the bacteria. This time scale is solidly within the Proterozoic, which is the correct answer.

For the second question,  it is important to look at what you are told about the information given in the graph. The graph gives you information about the percent of each gas in the atmosphere - nothing about the sheer amount of gases. So while the percent of the atmosphere that nitrogen accounts for decreases, you have no indication of whether that is because other gases are added or because the amount of nitrogen decreases - the answer to this question is therefore  "uncertain".

For the third question,  you are looking for a trend within the atmosphere that corresponds to the use of atmospheric carbon dioxide to produce oxygen. There are two possible places for this: where carbon dioxide starts to decrease and where oxygen starts to increase. While it is true that carbon dioxide decreases around 4.3 billion years ago, no oxygen is added to the atmosphere. When oxygen does begin to be produced, there is carbon dioxide present in the atmosphere (and the percent of carbon dioxide in the atmosphere does decrease slightly). This event occurs around 2.4 billion years ago.

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The figure above shows the historic composition, as percentages, of

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