6. (10 points) In the $6 \times 6$ grid on the right, each cell can only be filled with one of the letters $A, B, C, D, E, F$, with the requirement that no letter repeats in any row, column, or in any of the $2 \times 3$ rectangles marked with bold lines. Therefore, the letters in the four middle cells of the fourth row, from left to right, are ( )
A. $E, C, D, F$
B. $E, D, C, F$
C. $D, F, C, E$
D. $D, C, F, E$
【Analysis】First, analyze using the method of elimination and the uniqueness method. First, determine the number in the second column of the first row as $A$ according to the magic square rule. Following this line of thought, fill in the rest of the numbers.

【Solution】Solution: According to the problem, we know:
First, use the elimination method to look at the first grid. The first column cannot have $A$, and the second row cannot have $A$. Therefore, $A$ can only be in the second column of the first row.

The magic square rule and the method of elimination determine that the fourth column of the third row is also $A$;
The number in the fourth column of the fourth row is $C$;
Then the number in the fourth column of the fifth row is $F$;
So the number in the fourth column of the second row is $B$;
Continuing the reasoning, we get:
Therefore, the answer is: $C$.