At which point will a SmartPool license be required?

An Isilon customer has an 8-node cluster of older X-Series nodes.

They plan to upgrade the nodes over the next three years in the following stages:

– Stage 1: Add 2 X-Series nodes to meet performance growth

– Stage 2: Add 4 NL-Series nodes to meet archive capacity growth

– Stage 3: Add 10 New X-Series nodes as a hardware refresh

– Stage 4: Remove older X-Series nodes

At which point will a SmartPool license be required?
A . Stage 1
B . Stage 2
C . Stage 3
D . Stage 4