Final answer:
The key that you load in the 119 radio is frequency (option A). In FM radio, the carrier wave at the station's basic frequency is modulated in frequency by the audio signal without changing its amplitude.
Step-by-step explanation:
The key that you load in the 119 radio is frequency (option A).
In FM radio, the carrier wave at the station's basic frequency is modulated in frequency by the audio signal without changing its amplitude. This modulation of the carrier wave allows us to tune in to different radio stations by adjusting the frequency of the radio.