A call-back system requires that a user with an id and password call a remote server through a
dial-up line, then the server disconnects and:

A.
dials back to the user machine based on the user id and password using a telephone number from
its database.

B.
dials back to the user machine based on the user id and password using a telephone number
provided by the user during this connection.

C.
waits for a redial back from the user machine for reconfirmation and then verifies the user id and
password using its database.

D.
waits for a redial back from the user machine for reconfirmation and then verifies the user id and
password using the sender’s database.

Explanation:
A call-back system in a net centric environment would mean that a user with an id and password
calls a remote server through a dial-up line first, and then the server disconnects and dials back to
the user machine based on the user id and password using a telephone number from its database.
Although the server can depend upon its own database, it cannot know the authenticity of the dialer
when the user dials again. The server cannot depend upon the sender’s database to dial back as
the same could be manipulated.