In the diagram, a 4\times4 grid is to be filled so that each of the digits 1,2,3, and 4 appears in each row and each column. The 4\times4 grid is divided into four smaller 2\times2 squares. Each of these 2\times2 squares is also to contain each of the digits 1,2,3 and 4. What digit replaces P?
Answer Choices
A. 1
B. 2
C. 3
D. 4
E. The digit cannot be determined