Explanation: The first detector in a superheterodyne receiver is precisely its mixer stage. Its core function (A) is to combine the incoming radio frequency (RF) signal with a signal from the receiver's local oscillator (LO). This mixing process generates new frequencies, specifically the sum and difference frequencies of the two inputs, one of which is the desired intermediate frequency (IF). For this frequency conversion, or mixing, to occur, the device must (B) be operated on the non-linear portion of its characteristic curve. A perfectly linear device would simply pass or amplify the input signals without creating new frequencies. Non-linearity is crucial for producing the intermodulation products, including the sum and difference frequencies (the IF) that are essential for the superheterodyne principle. Thus, both statements accurately describe the necessary operation and function of the first detector.