6. In the complex plane, four complex numbers are located at the four vertices of a square. Three of the complex numbers are $1+2 \mathrm{i}$, $-2+\mathrm{i}$, and $-1-2 \mathrm{i}$. The fourth complex number is
A. $2+\mathrm{i}$
B. $2-\mathrm{i}$
C. $1-2 \mathrm{i}$
D. $-1+2 \mathrm{i}$
6. B $\quad 1+2 \mathrm{i}+(-1-2 \mathrm{i}+2-\mathrm{i})=2-\mathrm{i}$. 

Translate the above text into English, keeping the original text's line breaks and format, and output the translation result directly.

6. B $\quad 1+2 \mathrm{i}+(-1-2 \mathrm{i}+2-\mathrm{i})=2-\mathrm{i}$.