Two Waves with the Same Frequency and Wavelength but the Amplitudes

Multiple Choice

Two waves with the same frequency and wavelength but the amplitudes A₂ = 3A₁ are 4 $\pi$ rad out of phase. The amplitude of the resultant wave is

A) zero.
B) A₁.
C) 2A₁.
D) 3A₁.
E) 4A₁.