Five unit squares are arranged in the coordinate plane as shown, with the lower left corner at the origin. The slanted line, extending from (a, 0) to (3,3), divides the entire region into two regions of equal area. What is a ?

**Answer Choices**

A. \dfrac{1}{2}

B. \dfrac{3}{5}

C. \dfrac{2}{3}

D. \dfrac{3}{4}

E. \dfrac{4}{5}