IKMC 2009 - Grades 5-6 Question 30

We want to write in each square in the grid one of four letters A, B, C and D, so that neighbouring squares always have different letters (squares which share the same corner point also count as neighbouring squares). Some of the squares are already filled. Which letter should be written in the shaded square?



| | | | | |
|---|---|---|---|---|
| A | B | | C | D |
| | | | | |
| | | | | |
| | | | | ? |