Which is the best recommendation to ensure a short downtime for the users during upgrading the access layer switches?

An administrator of a company has concerns about upgrading the access layer switches. The users rely heavily on wireless and VoIP telephony .

Which is the best recommendation to ensure a short downtime for the users during upgrading the access layer switches?
A . Install the in-service software upgrade (ISSU) feature with clustering enabled
B . Install AOS-CX 6300 or 6400 switches with always-on POE
C . Implement VSF on the AOS-CX access switches
D . Implement VSX on the AOS-CX access switches

Answer: B