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