If Two Identical Waves with a Phase Difference of 6 $\pi$

If two identical waves with a phase difference of 6 $\pi$ are added,the result is

A) a wave with the same frequency but twice the amplitude.
B) a wave with the same amplitude but twice the frequency.
C) a wave with zero amplitude.
D) a wave with zero frequency.
E) This problem cannot be solved without knowing the wavelengths of the two waves.