在正方体\(ABCD-A _{1} B _{1} C _{1} D _{1}\)中，棱\(AB\)，\(A _{1} D _{1}\)的中点分别为\(E\)，\(F\)，则直线\(EF\)与平面\(AA _{1} D _{1} D\)所成角的余弦值为\((\:\:\:\:)\)
A. \( \dfrac { \sqrt {5}}{5}\)
B. \( \dfrac {2 \sqrt {5}}{5}\) C. \( \dfrac { \sqrt {6}}{6}\) D. \( \dfrac { \sqrt {30}}{6}\)
