Digital SAT Reading and Writing Practice Question #271

In forest ecosystems, mycorrhizal networks allow trees to share resources such as carbon. Ecologist Suzanne Simard and her colleagues conducted an experiment using Douglas-fir trees to determine whether these networks exhibit kin selection—favoring genetically related trees. They discovered that when an older 'hub tree' was shaded to reduce its own photosynthesis, it did not significantly draw carbon from surrounding trees. However, when the researchers shaded younger seedlings, older hub trees transferred substantially more carbon to them. Crucially, the researchers noted that this elevated transfer occurred only when the shaded seedlings were grown from the hub tree's own seeds, rather than seeds from an unrelated Douglas-fir. Which choice most logically completes the text?