Answer:
The most correct way to write this would be U.S.,
Explanation:
I have a few opitions which are all techincally grammaticlly correct.
First, the radio signals came from the United States; then they came from Canada. (More formal)
...came from the U.S. but then.... (also grammatrically correct)
...from the U.S., then from ....
hope this helps :)
