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Explanation:
A triangle is only possible if you can add two sides and have that sum be larger than the third side. This is the triangle inequality theorem.
Choice A is ruled out because the second and third sides add to 7+7 = 14, but that's not larger than the first side 15.
Choice B is a similar story: 6+8 = 14 is not larger than 14 (the first side)
Choice C as well: 6+7 = 13 is not larger than 14
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Choice D is the answer because
15+7 > 9
15+9 > 7
7+9 > 15
These three inequalities show that picking any two sides of this triangle, then adding them up, leads to a sum larger than the third side.
