Explanation: The mixer tube in a superheterodyne receiver is known as the first detector because it's the initial stage where the incoming radio frequency (RF) signal is processed to extract meaningful information, specifically by converting its frequency. The mixer combines the incoming RF signal with a locally generated signal from the local oscillator (LO). This process, called heterodyning, produces new frequencies, including the sum and difference of the input frequencies. The difference frequency is chosen as the fixed Intermediate Frequency (IF). This conversion to a constant IF simplifies subsequent amplification and filtering. The "second detector" (B) is responsible for demodulating the IF signal to recover the original audio or data. The "beat frequency oscillator" (C) is typically used for CW and SSB reception, introducing a beat frequency to the IF signal to make it audible or understandable, and is a separate component from the mixer. Therefore, the mixer serves as the first stage to "detect" and transform the signal's frequency.