Answer:
Third Box: The significance of the central object in this poster is that it exclaims that the United States is in danger of being invaded. This is significant as the posters purpose is the drum up support for US involvement in WWII.
Seventh Box:
Nature of the Enemy, as it displays the enemies goal of conquering the United States/the world.
Civil defense, as it is a call to action about directly protecting the United States.
Step-by-step explanation: