4. If in the sequence $\left\{a_{n}\right\}$, $a_{n}=\frac{100^{n}}{n!}$, then
A. $\left\{a_{n}\right\}$ is an increasing sequence
B. $\left\{a_{n}\right\}$ is a decreasing sequence
C. It becomes decreasing from a certain term
D. It becomes increasing from a certain term
4. Choose C.

Brief solution: Since $a_{n}>0$, we have $\frac{a_{n}}{a_{n-1}}=\frac{100^{n}(n-1)!}{100^{n-1} n!}=\frac{100}{n}$, when $n \geqslant 100$, $a_{n-1} \geqslant a_{n}$, it is decreasing.