Two identical waves meet exactly out of phase, crest lined up with trough. What is the result?

1. Crest lined up with trough means the two disturbances point in opposite directions everywhere. 2. Adding equal and opposite disturbances gives zero at every point. 3. That is pure destructive interference, and the waves completely cancel. 4. Twice the amplitude is the constructive case, which needs the waves in phase instead. 5. Superposition adds displacements and never alters the wavelength, so the last option is wrong on both counts. _Source: OpenStax College Physics (CC BY 4.0), Ch 16 "Oscillatory Motion and Waves", section 16.10 Superposition and Interference_