A circular spinner is divided into 4 sections, as shown. The angles at the centre of the circle in the sections labelled Green and Blue each measure 90^{\circ}. An arrow is attached to the centre of the spinner. The arrow is spun once. What is the probability that the arrow lands on either Red or Yellow?
Answer Choices
A. \frac{1}{8}
B. \frac{1}{4}
C. \frac{3}{8}
D. \frac{1}{2}
E. \frac{3}{4}