What is IP address shifting using a DIP?

What is IP address shifting using a DIP?

A.
When specific addresses in the DIP pool require port translation and others do not.

B.
When a one-to-one mapping of a range of private address to a range of public address is
needed.

C.
When a DIP has run out of addresses in the DIP pool and must “shift” to addresses from
another pool instead.

D.
When a DIP pool is “shifted” to use an address pool in the network range of the
secondary address instead of the primary address.