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.