Answer:
a. computers wait until the circuit is free before they send data.
Step-by-step explanation:
In Computer science, controlled access (X-ON/X-OFF) is typically the opposite of contention.
Controlled access is mainly used by computer networks that are being managed by a host mainframe computer system. X-ON is ready to receive while X-OFF isn't ready to receive.
With contention, computers wait until the circuit is free before they send data. It generally prevents collision between computer systems.