If I have only one domain, which servers should be Global Catalog (GC) servers?

A. If you have just one domain, Microsoft recommends that you make all the domain controllers (DCs) GC servers so that your network won't incur any extra space usage or processing. In essence, the infrastructure Flexible Single Master Operation (FSMO) role still checks the GC for many operations. By making all DCs GC servers, you can spread the FSMO's request load to all DCs and prevent one DC from asking another DC for information that the first DC already has. Although the FSMO can't typically reside on a GC, you won't encounter any problems as long as only one domain exists because the FSMO won't need to keep track of any external domain objects.