What is the primary reason for making an adjustment to the queue depth?

What is the primary reason for making an adjustment to the queue depth?

A.
number of LUNs presented to the HBA changes

B.
average size of I/O blocks going through the HBA changes

C.
read/write ratio of I/O going through the HBA changesread/write ratio of I/O? going through the HBA changes

D.
size of cache on the storage system changes