In the diagram below, the diameter of each of the two smaller circles is the radius of the larger circle. If the two smaller circles have a combined area of 1 square unit, then what is the area of the shaded region, in square units?
Answer Choices
A. \dfrac{1}{4}
B. \dfrac{1}{3}
C. \dfrac{1}{2}
D. 1
E. \dfrac{\pi}{2}