How does Isilon SnapshotIQ use storage for snapshots?

How does Isilon SnapshotIQ use storage for snapshots?
A . Through Changed Block Tracking and only changed data blocks are saved
B . Works on the file level and saves the entire file no matter what has changed
C . Works with 4 KiB blocks and will always use that amount of storage, no matter what has changed
D . Through Changed Block Tracking and always uses a 4 KiB block, even if the data changed is less than that amount

Answer: A