5. (10 points) The smallest natural number whose digits sum to 218 is an $n$-digit number, then $n=(\quad)$
A. 22
B. 23
C. 24
D. 25
【Analysis】To make this number as small as possible, the number of digits should be as few as possible, and each digit should preferably be the number 9; solve accordingly.

【Solution】Solution: To make this number as small as possible, the number of digits should be as few as possible,
Therefore, each digit should preferably be the number 9,
$$
218=9 \times 24+2
$$

Therefore, the smallest number is 224 nines,
So, the smallest natural number whose digit sum equals 218 is an $n$-digit number, then $n=24+1=25$; hence, the answer is: D.