In general, an economic recession refers to when a country or state is undergoing a period of slow economic growth (sometimes the growth is negative). It is less severe than a depression.
Take off D; the North had won some battles up to that point. Rip off A; the South would never let go of slavery. Take off B, since the Union had some wins already. You can now clearly see where the answer is.
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D.new theories reflected changes that were happening in society at large
A traditional story about travels or adventures can be Robert Louis Stevenson's Treasure Island. It speaks of a group of pirates in search of a lost treasure and certainly includes travels and adventures.