VTMO 2026 - Grade 10 - Semifinal - SET 1 - Self-study Question 2

Let $X=\{a_1,a_2,\ldots,a_{100}\}$ be a set of 100 real numbers. Define $D=\{a_i-a_j\mid a_i,a_j\in X,\ i\ne j\}$. What is the smallest possible number of elements of $D$?