6. 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 $2 \times 3$ rectangular blocks outlined with thick lines. Therefore, the letters in the middle four cells of the first row, from left to right, are ( ).
(A) $E, C, D, F$
(B) $E, D, C, F$
C )
$D, F, C, E$
(D) $D, C, F, E$
【Answer】 $\mathrm{C}$
【Key Points】Number Array Diagram: Sudoku
【Analysis】Each row, each column, and each $3 * 2$ bold-lined square must contain $\mathrm{A} 、 \mathrm{~B} 、 \mathrm{C} 、 \mathrm{D} 、 \mathrm{E} 、 \mathrm{~F}$ exactly once. Choose a suitable position and try it to quickly get the answer. The following provides one solution:
As shown in the figure, the first column and the second row already have $\mathrm{A}$, so the $\mathrm{A}$ in the top-left $3 * 2$ bold-lined square can only be placed in the second column; since the first and second columns already have $\mathrm{A}$, the $\mathrm{A}$ in the bottom-left $3 * 2$ bold-lined square can only be placed in the third column; since the fifth column and the fourth row already have $\mathrm{A}$, the $\mathrm{A}$ in the middle-right $3 * 2$ bold-lined square can only be placed in the fourth column; since the fifth row and the fifth column already have $\mathrm{A}$, the $\mathrm{A}$ in the bottom-right $3 * 2$ bold-lined square can only be placed in the sixth column; and so on, all numbers can be filled in.