Part C

Why are the two$-$source interference equations not valid for an observer on a road far away from two neighboring radio towers for different radio stations?

sources emit at different frequencies $($i$.$e$.,$ not monochromatic sources$)$

incoherent sources

observed from a distance similar to or smaller than the separation between the sources

$1$ only $2$ only $3$ only $1$ and $2$ only $1$ and $3$ only $2$ and $3$ only $$all three