Why did the publication of the Zimmermann telegram heighten Americans’

Why did the publication of the Zimmermann telegram heighten Americans’ sense of a threat to their national security?

A) It proposed an alliance between Germany and Mexico if the United States entered World War I.
B) It urged Mexico to launch a surprise attack on the United States.
C) It revealed that German U-boats had been ordered to patrol off the coast of the United States.
D) It revealed Germany’s plans to resume unrestricted submarine warfare.

ANSWER
A