Question 21·Hard·Command of Evidence
Ablation Rates for Three Elements in Cosmic Dust, by Dust Source
| Element | SPC | AST | HTC | OCC |
|---|---|---|---|---|
| iron | 20% | 28% | 90% | 98% |
| potassium | 44% | 74% | 97% | 100% |
| sodium | 45% | 75% | 99% | 100% |
Earth’s atmosphere is bombarded by cosmic dust originating from several sources: short-period comets (SPCs), particles from the asteroid belt (ASTs), Halley-type comets (HTCs), and Oort cloud comets (OCCs). Some of the dust’s material vaporizes in the atmosphere in a process called ablation, and the faster the particles move, the higher the rate of ablation. Astrophysicist Juan Diego Carrillo-Sánchez led a team that calculated average ablation rates for elements in the dust (such as iron and potassium) and showed that material in slower-moving SPC or AST dust has a lower rate than the same material in faster-moving HTC or OCC dust. For example, whereas the average ablation rate for iron from AST dust is 28%, the average rate for ______
Which choice most effectively uses data from the table to complete the example?
For SAT questions that ask which choice best uses data from a table to complete a sentence, first restate what the sentence is trying to show (for example, a comparison between two groups). Then identify exactly which variable(s) must stay the same (such as the same element, group, or category) and which must change (such as source type or condition). Next, go to the table and pull the specific numbers that match those criteria, and finally check each answer choice against both the data and the idea in the sentence. Eliminate any choice that changes the wrong variable or contradicts the stated relationship, even if its number appears in the table.
Hints
Focus on the comparison
Read the sentence around the blank: it is contrasting dust from slower sources (SPC or AST) with dust from faster sources (HTC or OCC). What kind of relationship between their ablation rates is it describing?
Keep the element the same
The example starts with “iron from AST dust is 28%.” To make a clear comparison, should the blank talk about the same element or a different one? Check which choices keep the element the same.
Use the correct source type
The second part of the example needs a dust source that fits the “faster-moving” category from the passage. Which answer choices use HTC or OCC, and how do their ablation rates compare to 28% in the table?
Step-by-step Explanation
Understand the claim being illustrated
The sentence says that slower-moving SPC or AST dust has a lower ablation rate than the same material in faster-moving HTC or OCC dust. The example begins: “whereas the average ablation rate for iron from AST dust is 28%...”
So the blank must complete this contrast by giving a higher ablation rate for iron from a faster source (HTC or OCC).
Identify the needed data in the table
From the table:
- We already use iron from AST: 28%.
- The “faster” sources we are told about are HTC and OCC.
Now look for the ablation rates for iron (same element) from HTC and OCC in the table and compare them to 28%.
Match the correct option to the data and idea
The table shows:
- Iron from HTC dust: 90%
- Iron from OCC dust: 98%
Among the answer choices, the only one that both mentions iron and comes from a faster source (HTC or OCC) is “iron from HTC dust is 90%.” This correctly shows that iron in slower AST dust (28%) has a lower ablation rate than the same material (iron) in faster HTC dust (90%), so the correct choice is “iron from HTC dust is 90%.”