Two sides of a triangle have lengths 10 and 15. What must be true about the length of the third side? A) less than 25 B) less than or equal to 25. C) equal to 25. D) more than 25.

In each triangle the sum of any two sides must be greater than the lenght of the other side - otherwise the triangle would be "open" or would only be a line. Since the sum of the two sides is 10+25=25, the third side must be longer than 25, so the correct answer is D.