Hi Fozzzy, now I think I understand the question better. Thanks for clarifying.
In this sentence: The Iroquois were primarily planters, but supplementing their cultivation of maize, squash, and beans with fishing and hunting
The author is implying that the Iroquois were primarily this, but also did a little of that. The "ing" modifier here (supplementing) modifies the entire previous sentence (the Iroquois were primarily planters), and would indicate a continuation of the same though. I.E. Ron was a strong student, graduating near the top of his class.... The "graduating" necessarily continues the same logic, and thus doesn't show contrast.
Now compare to "Ron was a strong student, but graduated near the bottom of his class due to a lack of focus". This shows the contrast of saying one thing, but then demonstrating another. In the example you've provided, the choice that properly shows this contrast is answer choice D, so that should be the correct answer.
Hope this helps!
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