资料题：People under stress often have digestive problems, because（  ）。

Because Ireland is an island geographically near the mainland of the United Kingdom, English rulers have fought since the Middle Ages to retain political control over it.Attracted by the lush farmland, English and Scottish landowners settled there, and in time of famine or political unrest, the local workers suffered, while their landlords were cushioned by their wealth.The history of modern Ireland is, in fact, largely a story of antagonism(斗争) and resentment between the Irish and their English and Scottish rulers.Since the 1920’s, Ireland has been divided into two parts: Northern Ireland (Ulster) and the Republic of Ireland (Eire).The north is till part of the United Kingdom and is predominantly Protestant; the south is an independent republic and is mainly Catholic.The majority in Ulster accepted this political compromise, but the active and mainly Catholic minority are fighting for union with the independent republic of Southern Ireland.The IRA, the Irish Republican Army, have mounted bombing campaigns on military and civil targets in Ulster and England, they have sent letter-bombs to public figures, they have shot fellow Irishmen who support the British or belong to opposing.and now equally militant Protestant groups.As a result of this, the British have stationed an army in Belfast, the RA have been outlawed, and several of them have spent many years in prison or have died in support of their cause.Whether this level of violence and repression is justifiable, and whether the violence that could result from political change would be worthwhile are the controversial issues that divide everybody involved.