Rejecting an Ambiguous Main-Idea Key
You are validating keys for an unseen reading-comprehension passage. The passage reads: 'A city's tree-cover audit found that street trees can lower surface temperature by 2-3°C within 20 metres, but only where soil volume is adequate. Saplings planted in pits under one cubic metre survived at 40% after three years; those in connected trenches survived at 78%. The report therefore recommends connecting planting pits rather than counting trees alone. Critics note that trenching is expensive and can damage pavements, so the report suggests prioritising streets with wide verges and high heat risk.' Two candidate keys for a 1-mark main-idea item: Key A: 'Trees cool streets.' Key B: 'The cooling benefit of street trees depends on planting conditions, not just tree numbers.' A second item asks: 'The word "adequate" is closest in meaning to...' with options: sufficient / modern / equal / narrow. The provisional key is B and sufficient.
1. Is Key B the only defensible main idea? Explain why Key A fails even though it is true.
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Key B is defensible; Key A is true but too broad to be the main idea. The passage's organising claim is conditional: cooling occurs 'but only where soil volume is adequate', and the survival figures compare planting methods. A student who chooses A has picked up a true supporting fact and mistaken it for the whole passage's point. B captures the contrast between counting trees and connecting pits. If the item is 1 mark, B is the only key that rewards reading the passage's structure rather than its topic.
2. Write an inference question from the passage whose answer is not stated in one sentence, with a 1-mark key and a reason.
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Inference question: 'Why does the report recommend connected trenches rather than simply planting more trees?' Key: 'Because survival and therefore cooling depend on adequate soil volume, and trenching provides it; planting more trees in small pits would not reliably produce the same benefit.' The answer is not in one sentence: it requires linking the 40%/78% survival figures to the soil-volume condition and the recommendation. A 1-mark key should accept the link, not just 'trenches are better'.
3. Evaluate the vocabulary item: does it test context or dictionary recall?
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The vocabulary item is context-sensitive but not perfect. 'Adequate' in 'soil volume is adequate' means sufficient for the tree's needs; 'sufficient' is the key. 'Equal' is a tempting distractor because adequate can suggest meeting a standard, but the passage is about volume, not equality. 'Modern' and 'narrow' are not supported. The item tests contextual synonymy reasonably well, though a stronger stem would quote the phrase: 'soil volume is adequate' rather than the bare word.