Tính \( \left( { \overrightarrow a , \overrightarrow b } \right) \)biết \( \overrightarrow a . \overrightarrow b = - \dfrac{1}{2} \left| { \overrightarrow a } \right|. \left| { \overrightarrow b } \right| \), \( \left( { \overrightarrow a ; \, \overrightarrow b \, \ne \overrightarrow 0 } \right) \)
A. \(135^\circ \)
B.\(60^\circ \)
C.\(150^\circ \)
D.\(120^\circ \)