Which of the following best describes Four Wave Mixing (FWM)?

FWM is a linear impairment that produces crosstalk four adjacent wavelengths are used.

FWM is a non-linear impairment that produces crosstalk at the sum and difference of two wavelengths.

FWM is a non-linear impairment resulting from varying velocities at different frequencies.

FWM is a linear impairment resulting from pulses broadening over ultra-longdistances