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 :) 
 
        
             
        
        
        
Answer:
it the first one is the answer for this question 
Explanation: