Each side of square ABCD is 8. A circle is drawn through A and D so that it is tangent to BC. What is the radius of this circle?

Answer Choices
A. 4
B. 5
C. 6
D. 4\sqrt2
E. 5.25
Each side of square ABCD is 8. A circle is drawn through A and D so that it is tangent to BC. What is the radius of this circle?
Answer Choices
A. 4
B. 5
C. 6
D. 4\sqrt2
E. 5.25