All the numbers 1,2,3,4,5,6,7,8,9 are written in a 3 \times 3 array of squares, one number in each square, in such a way that if two numbers are consecutive then they occupy squares that share an edge. The numbers in the four corners add up to 18 . What number is in the center?
Answer Choices
A. 5
B. 6
C. 7
D. 8
E. 9