The question seems quite straightforward.
Conclusion is that the origin of the next cup discovered can be easily determined.
Going through the answer choices we can find that the argument clearly assumes that the next cup to be discovered would be made of porcelain. Although we can also apply the LEN method : The next cup to be discovered need not be porcelain. This clearly kills the argument. So "C" is the right answer.
Now we have to select a choice which as a fact could strengthen the assumption. We can see that choice "A" clearly strengthens this assumption.
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