[单选] Ireland has had a very difficult history． The problems started in the 16th century when English rulers tried to conquer （征服） Ireland． For hundreds of years， the Irish people fought against the English． Finally， in 1921， the British government was forced to give independence to the south of Ireland． The result is that today there are two "Irelands"． Northern Ireland—in the north， is part of the United Kingdom． The republic of Ireland—in the south， is an independent country． In the 1840s the main crop， potatoes was affected by disease and about 750，000 people died of hunger． This， and a shortage （短缺） of work， forced many people to leave Ireland and live in the USA， the UK， Australia and Canada． As a result of these problems， the population fell from 8．2 million in 1841 to 6．6 million in 1851． What is the first paragraph mainly about？