I talked about this in the setup. We’re always missing one of W and Z. They’re both topazes.
We always need two topazes. If we can only have one of W/Z, then we need at least one of X/Y to get two topazes.
D is CORRECT.
If you’re unsure about any of the other answer choices, the surest way to eliminate them is to draw scenarios as counterexamples.
It helps to look back at past questions; all of my scenarios below come from earlier questions.
This scenario shows that A and C are wrong:
We don’t need rubies, and we don’t need F or Z.
This scenario shows that B is wrong:
We can have three rubies.
This scenario shows that E is wrong:
We don’t need X or Z.
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