Why did the u. s. join the fighting in world war 1 ?

There are traits like blood type and eye color, and others like speaking english or having a scar. what are the terms used to describe these different types of traits? how are each defined?

When paul realizes no one is coming to the rwandan refugees, he tells them there is only one way for them to get . what is it?

When recursive formula can be used to determine the total amount of money earned in any year based on the amount earned in the previous years?