Extra exam question E7E08

WhyThe principal frequencies appearing at the output of a mixer are the two input frequencies along with their sum and difference frequencies. A mixer's output principally contains the two original input frequencies along with their sum and their difference, the fundamental mixing products that a mixer stage is built to generate, one of which (usually the difference, in a receiver) is then selected by filtering, answer C. It doesn't simply produce frequencies at two and four times the input, doesn't produce a square-root-of-product frequency, and doesn't produce output at 1.414 and 0.707 times the input, none of those describe how mixer products are actually generated.