6. Xiao Ming and Xiao Hua are playing a game, with the scoring rules as follows: at the beginning, each person's score is 1 point; after each round, they multiply their score by 3. After the game ends, the difference between Xiao Ming's score and Xiao Hua's score is exactly a positive integer multiple of 675. Then, how many more rounds did Xiao Ming play compared to Xiao Hua at least?
(A) 15
(B) 20
(C) 25
(D) 30
6.B.

Let Xiaoming jump $m$ times, and Xiaohua jump $n$ times. From the problem, we have $3^{m}-3^{n}=675 k\left(k \in \mathbf{N}_{+}\right)$,
which means $3^{n}\left(3^{m-n}-1\right)=3^{3} \times 5^{2} \times k$.
Therefore, $3^{m-n}-1$ is a multiple of 25. Then $\min (m-n)=20$.