In the diagram, 18 identical 1 \times 2 rectangles are put together to form a 6 \times 6 square. Part of the square is shaded, as shown.
What percentage of the area of the 6 \times 6 square is shaded?
Answer Choices
A. 50 \%
B. 67 \%
C. 75 \%
D. 33 \%
E. 25 \%