Which fact helps you prove the isosceles triangle theorem, which states that the base angles of any isosceles triangle have equal measure?

In an isosceles triangle, the two legs are equal. If the base angles were unequal, the larger base angle would be opposite one of those legs, and the smaller base angle would be opposite the other. Choice D says the side opposite the larger angle must be longer, contradicting that the legs are equal. Therefore, the base angles must have equal measure.

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