What is possible reason for this issue?

A Dell EMC NetWorker administrator is configuring a staging resource to automatically move sets that were saved in the past 7 days to another volume. However, the administrator discovered that no source devices were available.

What is possible reason for this issue?
A . All tape devices are in write-enabled mode only
B . Source volumes on the library are not completely full
C . No disk-based devices were configured
D . Automatic staging to a different device type is not supported

Answer: C