Answer:
The inference is supported by the excitement the men feel when they discover that the message was sent by Zimmermann, the German foreign secretary.
Step-by-step explanation:
In the excerpt the message seems out of the ordinary right away. They give the example that any message signed with a number in the 90000 range indicates that it was sent by someone important. This is then confirmed when they find out the number is assigned to Zimmermann. It then says that this realization makes Montgomery and de Grey very excited.