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The main function of a modem in a network is to convert between digital and analog signals. This conversion is essential because data is typically generated and used in a digital format, whereas many transmission mediums, such as traditional telephone lines, use analog signals for communication.

When a device, such as a computer, communicates over these analog lines, the modem transforms the digital data from the device into an analog signal that can travel over the line. Conversely, when data is received, the modem converts the incoming analog signal back into digital data that the device can understand. This bidirectional conversion enables effective communication between digital devices and analog communication systems, which is critical for internet access and various networking functions.

The role of a modem is fundamental to connecting different types of networks and facilitating communication, which is why option B is the correct choice.