5. If the sequence $\left\{a_{n}\right\}$, where $a_{n}=\frac{100^{n}}{n!}$, then the correct answer is ( ).
A. Increasing sequence
B. Decreasing sequence
C. From a certain term onwards, it is a decreasing sequence
D. From a certain term onwards, it is an increasing sequence
5. C

Since $a_{n}>0$, we have $\frac{a_{n}}{a_{n-1}}=\frac{100^{n}(n-1)!}{100^{n-1} n!}=\frac{100}{n}$, thus when $n \geqslant 100$, $a_{n-1} \geqslant a_{n}$, the sequence is decreasing.