What might cause the upgrade to be unsuccessful?

An administrator is upgrading an ESXi 3.5 host to ESXi 5.x with an intermediate upgrade to 4.0
using Update Manager. The host has a single Intel Xeon processor, 4GB of RAM, and a VMFS
datastore on a private, SAN-attached LUN. What might cause the upgrade to be unsuccessful?

A.
The ESXi host has 4GB RAM.

B.
The ESXi host has one processor.

C.
ESXi 3.5 is not supported for upgrade.

D.
Update Manager cannot upgrade a previously upgraded host.

Explanation:
Reference: http://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.vsphere.upgrade.doc_50%2FGUID-AA106086-E10D-456EB0B9-340D3D1462C3.html