Let $A$ and $B$ be two finite sets having $m$ and $n$ elements respectively such that $m \le n.$ $A$ mapping is selected at random from the set of all mappings from $A$ to $B$. The probability that the mapping selected is an injection is

• A
  $\frac{n!}{(n - m)! m^n}$
• B
  $\frac{n!}{(n - m)! n^m}$
• C
  $\frac{m!}{(n - m)! n^m}$
• D
  $\frac{m!}{(n - m)! m^n}$