3. a transmission system at a radio station uses a/an to convert a direct current into a high-frequency alternating current. a. transmitting antenna b. modulator c. demodulator d. oscillator

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

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o conteúdo de calor de um sistema é igual à entalpia apenas para um sistema que é constante