Which two circumstances might cause a DRS cluster to become invalid?

Explanation:
The DRS becomes invalid if:
1. A cluster might become overcommitted if a host fails.
2. A cluster becomes invalid if vCenter Server is unavailable and you power on virtual machines using a vSphere Client connected directly to a host.
3. A cluster becomes invalid if the user reduces the reservation on a parent resource pool while a virtual machine is in the process of failing over.
4. If changes are made to hosts or virtual machines using a vSphere Client connected to a host while vCenter Server is unavailable, those changes take effect. When vCenter Server becomes available again, you might find that clusters have turned red or yellow because cluster requirements are no longer met.
http://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.vsphere.resmgmt.doc_50%2FGUID-8C04BAA0-45BD-4484-BAB3-CE6FD24FEB0E.html
Which two circumstances might cause a DRS cluster to become invalid? (Choose two)
A. A reservation on a parent pool is reduced while a virtual machine is failing over
B. An ESXi host in the cluster has been placed into maintenance mode
C. A migration on a virtual machine is attempted while the virtual machine is failing over
D. Virtual machines have been powered on from a vSphere Client connected directily to an ESXi host
Correct Answer: AD
Explanation/Reference:
Pg 70 vsphere-esxi-vcenter-server-50-resource-management-guide.pdf
Invalid DRS Clusters
A cluster enabled for DRS becomes invalid (red) when the tree is no longer internally consistent, that is, resource constraints are not observed.
The total amount of resources in the cluster does not affect whether the cluster is red. A cluster can be red, even if enough resources exist at the root level, if there is an inconsistency at a child level.
You can resolve a red DRS cluster problem either by powering off one or more virtual machines, moving virtual machines to parts of the tree that have sufficient resources, or editing the resource pool settings in the red part. Adding resources typically helps only
when you are in the yellow state.
A cluster can also turn red if you reconfigure a resource pool while a virtual machine is failing over. A virtual machine that is failing over is disconnected and does not count toward the reservation used by the parent resource pool. You might reduce the reservation
of the parent resource pool before the failover completes. After the failover is complete, the virtual machine resources are again charged to the parent resource pool. If the pool’s usage becomes larger than the new reservation, the cluster turns red.
If a user is able to start a virtual machine (in an unsupported way) with a reservation of 3GHz under resource pool 2, the cluster would become red, as shown in the following figure.

Which two circumstances might cause a DRS cluster to become invalid? (Choose two.)
A.
DRS has been disabled on one or more ESXi hosts.
B.
An ESXi host has been removed from the cluster.
C.
A migration on a virtual machine is attempted while the virtual machine is failing over.
D.
Virtual machines have been powered on from a vSphere Client connected directly to an ESXi
host.