Which condition will cause a HA/DRS cluster to display a yellow health indicator?

Which condition will cause a HA/DRS cluster to display a yellow health indicator?

A.
One or more hosts in the cluster has entered maintenance mode.

B.
There are insufficient resources to satisfy the cluster’s requirements.

C.
A virtual machine has failed and cannot be restarted in the cluster.

D.
Virtual machine I/O activity in the cluster has reached or exceeded a critical level.