A system consists of n identical particles each of mass m. The total number of interactions possible is

n(n + 1) $\frac{1}{2} n(n+1)$ n(n - 1) $\frac{1}{2} n(n-1)$

$\frac{1}{2} n(n-1)$

Total number of interactions = ${ }^n C_2=\frac{n(n-1)}{2}$