the amplifier the battery the microphone the speaker
I gave my students this multiple choice question: A hearing aid is a small device that contains an amplifier, a battery, a microphone, and a speaker. Which part of a hearing aid functions to detect sounds in the user's environment?

Only one answer is correct among these 4 choices:
- the amplifier
- the battery
- the microphone
- the speaker

Could you tell me which one is correct?

the microphone