II. (Full marks 15 points) $f(n)$ is a strictly increasing function defined on $\mathbf{N}$ and taking integer values (if for any $x_{1}, x_{2} \in A$, when $x_{1}<x_{2}$, we have $f\left(x_{1}\right)<f\left(x_{2}\right)$, then $f(x)$ is called a strictly increasing function on $A$). When $m, n$ are coprime, $f(m n)=f(m) f(n)$, and $f(19)=19$. Try to find $f(f(19) f(98))$.