A prediction question about a study tests what follows if a proposed explanation is true. Start with the question to see whose explanation you’re asked to assume, then trace the cause to the result. What should change if that cause is removed or reduced? Keep your prediction limited to the study’s setting and the effect the explanation predicts. Wrong choices often add effects that the explanation never connects to the cause.
Hints
- Hint 1
The passage describes both a benefit of fallen leaves and a proposed reason for shedding them. Find the reason the colleague gives, since that’s the explanation the question asks you to assume.
- Hint 2
In the colleague’s explanation, how does nitrogen supply affect keeping leaves? Use that link to check what causes what in each outcome.
Step-by-step
Follow the proposed cause
Step 1Separate the cause from the benefit
Most trees in the species A forest kept their leaves; in the species B forest, many trees shed their leaves early. The fallen leaves later helped the soil stay moist, but what are we asked to assume? If the colleague’s explanation is correct tells us to predict from the colleague’s reason for leaf drop. The mulch benefit isn't automatically the cause.
- Step 2
Trace the nitrogen explanation
What sequence does the colleague propose? The phrase drought-reduced nitrogen released by soil microbes means drought leaves the trees with less nitrogen from microbes. Keeping leaves then becomes less worthwhile, so the trees shed them early. The nitrogen shortage comes before leaf drop in this explanation.
- Step 3
Predict a change in timing
What should happen if the nitrogen shortage eases? The colleague says low nitrogen makes it less beneficial for trees to maintain leaves. Easing the shortage should weaken the reason for early shedding. This prediction is about when leaves fall during drought, not what controls leaf drop in every season.
- Step 4
Test the predicted outcome
The outcome with supplemental nitrogen supplies what the colleague says drought takes away. Comparing similar trees with and without it during a drought means the nitrogen supply is what changes. The trees receiving the supplement should then delay leaf drop compared with those without it, which is the outcome this choice describes. Choice B.