Connecting two computers together using a serial cable:

The correct choice is that connecting two computers together using a serial cable requires a null modem since both computers are Data Terminal Equipment (DTE).

In a serial communication setup, devices are typically classified as DTE or Data Communication Equipment (DCE). Computers are considered DTE because they are the endpoints of data communication, while devices such as modems are classified as DCE. For direct connection between two DTE devices, a null modem is necessary to swap the transmit (TX) and receive (RX) lines, allowing the two computers to communicate with each other correctly.

Using a null modem cable essentially allows one computer's transmit line to connect to the other computer's receive line, thus facilitating proper data exchange. Without this configuration, the direct connection would mean both devices trying to send data on the same line, leading to communication failure.

The other options presented are based on misunderstandings of serial communication. The idea that double voltage would damage serial ports is incorrect, as standard voltage levels are designed to be within safe limits for data communication. Matching application layer protocols does not address the physical connection method needed for the two DTE devices. Lastly, a voltage inverter circuit is not necessary for connecting two computers directly with a serial connection since the DTE devices