Public address system and vacuum pump: what they have in common

1. Step 1. Public address system — has part — Amplifier. Amplifiers.
2. Step 2. Amplifier — has part — Triode. Tube amplifiers.
3. Step 3. Triode — uses — Vacuum. It works in a vacuum.
4. Step 4. Vacuum — is used in — Vacuum pump. The pump creates a vacuum.

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