Notice that the first two sentences are the reasons because they support the conclusion. An assumption is an unstated reason. It is a missing reason which is necessary in order to accept the conclusion. If you read an argument that featured an assumption and accepted the conclusion, then your brain filled in this unstated, but essential reason. It is important to identify key assumptions, because like all reasons, they are not always acceptable.
Every argument has one main conclusion.
It is one sentence and it is the proposition that the author is trying to prove . The conclusion can appear anywhere in an argument. It does not have to appear at the end. Often it will be the first sentence. An inference is a conclusion that the receiver draws.