Which of the following archiving methods only backs up data that has changed since the previous full backup？（）

An administrator is creating a redundant environment for a current data center. A backup site must be available if the entire data center site should become unavailable. To reduce costs， this backup site does not need to be up and running at all times ready for failover. It must contain equipment and the ability to restore data from the data center. 
Which of the following site types should be considered to meet these requirements？ （）