Alternates need to be listed at the same time as the entrants. The US put three alternate couples on their 4CC entry: Zingas/Kolesnik, Brown/Brown and Pate/Bye. They can only use those alternates. Even if all three of the original couples and one of the alternates were unable to compete, leaving the US with just two couples competing, they still would not be able to enter Bratti/Somerville at this point.