*Question T2B06:
What type of signaling uses pairs of audio tones?*  

*Correct Answer is  A: DTMF*  

### AI Response: (may contain errors)  

To understand the correct answer to the question about signaling that uses pairs of audio tones, let's break down each option:

### Correct Answer: DTMF (Dual-Tone Multi-Frequency)

- **Explanation**: DTMF is a signaling method used in touch-tone telephones. It works by generating pairs of audio tones, one from a low-frequency group and one from a high-frequency group. Each key on the keypad corresponds to a unique combination of these two frequencies, allowing for reliable signal transmission.

### Why the Other Choices are Incorrect:

- **CTCSS (Continuous Tone-Coded Squelch System)**:
  - CTCSS uses a single sub-audible tone (usually between 67 Hz and 250 Hz) to control the squelch of a receiver. It does not use pairs of tones, but rather a single tone to indicate that a transmission is intended for a specific group.

- **GPRS (General Packet Radio Service)**:
  - GPRS is a packet-oriented mobile data standard that allows for data transmission over cellular networks. It does not involve audio tones for signaling; instead, it uses digital data packets.

- **D-STAR (Digital Smart Technologies for Amateur Radio)**:
  - D-STAR is a digital voice and data protocol used in amateur radio. It employs digital modulation techniques and does not utilize pairs of audio tones for signaling.

### Summary

- **DTMF**: Uses pairs of audio tones for signaling (correct answer).
- **CTCSS**: Uses a single tone for squelch control.
- **GPRS**: Involves digital data transmission, not audio tones.
- **D-STAR**: Utilizes digital modulation, not audio tones.

This distinction is crucial for understanding the different signaling methods used in telecommunications and amateur radio.