Final answer:
A modem allows computers to communicate over telephone lines by converting digital data into analog signals for transmission, crucial for internet connectivity.
Explanation:
An expansion card that allows a computer to communicate over telephone lines is a Modem (Modulator-Demodulator).
Modems convert digital data from a computer into analog signals for transmission over telephone lines and vice versa. For example, a student used an S15C data terminal, connected to a computer center via a Bell System DATA Phone, which utilized a modem for communication.
Having a Modem is crucial for internet connectivity, especially in the past when dial-up connections were prevalent and essential for network communication.
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