For “most logically completes the text” items, use the words right before the blank to identify the relationship (here, “because” signals a cause). Then match the choice that restates the passage’s explicitly stated constraints, and avoid options that introduce a different-sounding limitation not mentioned in the text.
Hints
- Hint 1
Use the connector
The word “because” means the blank must give a cause explaining why tree rings + ice cores can’t achieve the 80,000-year extension.
- Hint 2
Reuse the passage’s stated limits
Identify the limitation for tree rings (time span) and the limitation for mountain ice cores (some glaciers can’t preserve yearly layers).
- Hint 3
Watch for near-miss science claims
Some choices will sound scientifically reasonable (melting, dating problems, polar ice sheets), but the correct answer must match the exact limitation stated in the passage.
Step-by-step
- Step 1
Determine what the blank must do
The sentence ends with “could not accomplish because ______,” so the blank must provide the reason tree rings plus ice cores still fall short.
- Step 2
Locate the two limitations stated in the text
The text directly states:
- Tree rings: they “rarely extend beyond 10,000 years.”
- Ice cores (from mountain glaciers): they can’t be taken everywhere because some glaciers are “too thin to preserve yearly layers.”
- Step 3
Eliminate choices that change the passage’s stated limitation
The correct option must match the passage’s specific obstacle for ice cores (thin glaciers can’t preserve yearly layers), not a different issue like melting, polar-only extraction, or general dating difficulty.
- Step 4
Choose the option that restates both stated constraints
The only choice that directly combines the passage’s two stated limits is:
tree-ring chronologies rarely extend beyond 10,000 years and some mountain glaciers are too thin to preserve yearly layers.