If Two Identical Waves with a Phase Difference of 3 $\pi$

Multiple Choice

If two identical waves with a phase difference of 3 $\pi$ 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 an intensity equal to the sum of the intensities of the two waves.
E) This problem cannot be solved without knowing the wavelengths of the two waves.