A transmission system at a radio station consists of essentially the following devices:
1. An oscillator, which converts a direct current into a high-frequency alternating current.
2. A transmitting antenna, which sends out radio waves having the same frequency as the current supplied by the oscillator.
3. A microphone, which picks up the sound waves and converts them into a suitable electric current with the same pattern as the sound waves.
4. A modulator, which enables the waves produced by the antenna to “carry” the pattern of the sound waves picked up by the microphone.
The answer is D. Oscillator