名词同义词组：
1. 困难：difficulty / handicap / hardship / complication / dilemma / plight
2. 将来：future / prospect / forthcoming
3. 障碍：obstacle / barricade / block / obstruction / snag / hurdle
动词同义词组：
1. 要求：demand / request / claim / require / call for
2. 下降：decline / decrease / degenerate / drop / fall / worsen / depress
3. 发现：find / discover / come across / encounter / expose / spot / recognize / uncover / notice / observe
4. 模拟：simulate / virtual / create / reproduce / imitate
形容词同义词组：
1. 人工的：artificial / fake / bogus / synthetic
2. 不可避免的：inevitable / unavoidable / destined /fated / inescapable
3. 普遍的：prevalent / widespread / common / systematic / general / popular