Nếu \(\sin a = - \frac{{\sqrt 2 }}{3}\) thì \(\sin \left( {a + \frac{\pi }{4}} \right) + \sin \left( {a - \frac{\pi }{4}} \right)\) bằng: A. \(\frac{2}{3}\). B. \(\frac{1}{3}\). C. \( - \frac{2}{3}\). D. \( - \frac{1}{3}\).

Nếu \(\sin a = - \frac{{\sqrt 2 }}{3}\) thì \(\sin \left( {a + \frac{\pi }{4}} \right) + \sin \left( {a - \frac{\pi }{4}} \right)\) bằng:

A. \(\frac{2}{3}\). B. \(\frac{1}{3}\). C. \( - \frac{2}{3}\).

D. \( - \frac{1}{3}\).