None of Cassatt's works is
Why this is right
We can recognize that this is a conditional answer and try to diagram it, so that we can ask ourselves, "Did the author make this (or the contrapositive of this) reasoning move?" This is a tricky statement since it involves a double negative: None of her works used a style not-exhibited in any of her known works. If we said, "None of my friends are not-happy to to get free food", it would mean the same as "All of my friends are happy to get free food". So this sentence is saying, "All of her works use a brush style that is exhibited in some of her known works." The contrapositive is much more appealing match for the argument's reasoning move: "If a work is painted in a brush style that is not-exhibited in any of Cassatt's known works, then it is not a work of Cassatt". We could also reference our knowledge that "No A's are B" is the same as "All A's are ~B". Thus, this is saying all of her works do not use a brush style that is not exhibited in any of her known works. Again we have a double negative at the end there, so it turns into a positive: "all of her works use a brush style that is exhibited in at least one of her known works". Our other technique on Necessary Assumption is the negation test. If we negate an answer and it becomes an objection, then we have a winning answer. The negation of None is Some (at least one). So the negation of this answer is saying, "At least one of Cassatt's works is painted using a brush style that is not exhibited in any of her known works". That would badly weaken the argument. The author thinks that the fact that this painting has a brush style that doesn't match any of her known works rules out the possibility of this painting being by Cassatt. Negating this answer says that there's at least one Cassatt painting that has such a trait (its brush style doesn't match any other Cassatt painting).
Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.