Geometry

Question Analysis:
This question is about the fundamental property of equilateral triangles regarding side lengths.
The main focus is on the basic definition of an equilateral triangle.

Key Concept Explanation:
By definition, an equilateral triangle has all three sides of equal length.

Step - by - Step Solution:
Based on the definition, we directly know that the sides of an equilateral triangle are equal.

Option Analysis:
Option A: Different lengths is incorrect as it contradicts the definition of an equilateral triangle.
Option B: Equal lengths is correct.
Option C: There is no such general relationship that sides are twice the length of the altitude, so this is incorrect.<...