6. A monkey is on a ladder with $n$ steps: climbing up and down, it either ascends 16 steps or descends 9 steps each time. If it can climb from the ground to the very top step, and then return to the ground, the minimum value of $n$ is ( ).
(A) 22
(B) 23
(C) 24
(D) greater than 24.
6. (C).

Assume the monkey climbs as follows: 0 $\nearrow 16 \searrow 7 \nearrow 23 \searrow 14 \searrow 5$ $\nearrow 21 \searrow 12 \searrow 3 \nearrow 19 \searrow 10 \searrow 1, \nearrow 17 \searrow 8 \nearrow 24 \searrow 15 \searrow 6 \nearrow$ $22 \searrow 13 \searrow 4$ Л $20 \searrow 11 \searrow 2$ オ゙ $18 \searrow 9 \searrow 0$. We get that the minimum value of $n$ is 24.

In fact, when $n<24$, the monkey's climbing method is unique. At the latest, when the monkey climbs to the 8th step, it can neither climb up nor down, which contradicts the problem statement. Therefore, the minimum value of $n$ is 24.