Read the passage first
The lesson below refers to this passage.
The Garden After Rain
Overview
An explanation often gives a mechanism and then limits its reach. Read both parts. A cause tells how one condition brings about another; a qualification tells when that explanation is insufficient or uncertain. A good main idea preserves the mechanism without turning one demonstration into a rule for every case.
In “The Garden After Rain,” repeated walking presses soil particles together, leaving fewer spaces for water movement. The steps form a chain: pressure changes structure, structure restricts movement, and water remains at the surface. Saying that traffic increases rainfall reverses the role of the observed rain, which was equal in the two beds. The later discussion of texture, slope, and deeper layers does not deny compaction. It explains why a puddle elsewhere cannot diagnose compaction by itself. “Permeable” consequently means allowing passage through the material, not simply fertile or good for plants.
“A Record in the Mud” requires a related caution. Pollen indicates plants, but different plants produce different amounts and grains can travel. Equal counts therefore do not establish equal areas of vegetation. Comparing another lake tests whether the pattern is local or more widespread; agreement strengthens the interpretation without making either column a photograph of the landscape. The photograph comparison deliberately limits the evidence's completeness.
When selecting a conclusion, name the observed result and list the conditions that could change its interpretation. For evidence questions, identify the exact claim being challenged: differing pollen production directly challenges a count-to-area conversion, whereas the sampling tube only explains collection. For structure questions, ask whether a paragraph illustrates a mechanism, supplies corroboration, or introduces limits. Transfer the method by checking direction, conditions, and evidence coverage before accepting a causal generalization.
Apply the method to a complete new context
Read the whole passage before choosing an answer.
Frost in the Low Corner
On clear, calm nights, the lowest corner of an orchard often frosts before the hillside above it. As the ground loses heat after sunset, the air touching it cools and becomes denser. That heavier air slides downhill and collects in the hollow, much as water would. Growers call such places frost pockets.
The explanation has limits. Wind mixes the air and can erase the difference between slope and hollow, and cloud cover slows the ground's loss of heat. A hedge across the bottom of a slope can even hold cold air above the hollow. A frosted corner therefore suggests pooling air, but a grower should check the night's wind and sky before blaming the shape of the land alone.
Which statement best expresses the central idea? A. Hollows frost first because the surrounding slopes block the sunlight they would otherwise receive. B. Cold air can pool in low ground, although wind, clouds, and barriers affect whether it does. C. The low corner of an orchard frosts on every clear night, whatever the weather or layout. D. Hedges cause most orchard frost by trapping warm air close to the trees on a slope.
The supported answer is B: Cold air can pool in low ground, although wind, clouds, and barriers affect whether it does. The first paragraph gives the mechanism, chilled denser air flowing downhill, and the second names conditions that weaken or block it. Choice C keeps the mechanism but turns it into a rule for every clear night, which the wind, cloud, and hedge examples contradict.
Before accepting a causal summary, check that it keeps both the mechanism and the conditions that limit it.