With contention:. a. computers wait until the circuit is free before they send data
b. the server or front end processor works consecutively through a list of clients to determine who should have access to the media
c. the front end processor must wait for a response from the polled client or terminal
d. one computer starts the poll and passes it to the next computer on the multipoint circuit
e. there is never a chance for collision, or two computers trying to send data at the same time

a. computers wait until the circuit is free before they send data.

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.

i think its controle not shift

i do not get the question.