If a, b, c, d are positive real numbers such that a, b, c, d form an increasing arithmetic sequence and a, b, d form a geometric sequence, then \dfrac{a}{d} is
Answer Choices
A. \dfrac{1}{12}
B. \dfrac{1}{6}
C. \dfrac{1}{4}
D. \dfrac{1}{3}
E. \dfrac{1}{2}