答案：1-5ADBDB
1.选A 四个选项都有“绳索”这个意思，但只有thread和hang by构成习语：hang by a thread千钧一发，危在旦夕
2.选D be+ prep+n(表动作的名词)来表示“被动”而under discussion是习惯搭配
3.选B descend 指人，物体从某一高度/斜面上“下来/降”On turning the corner, we saw that the road descended sharply.
decline 强调权利，繁荣，成就，健康状况，体力，记忆，数量的“减少，衰落”His influence declined after he lost the election.
ascend渐渐上升，descend的反义词。We watched the balloon ascend higher and higher.
plunge使突然前倾或下降 The sudden application of the brakes plunged him forward.
enhance 增加，提高The moonlight enhanced the beauty of the scene.
4.选D gear to使……适合，配合We must gear the amount of products we make to the increase in public demand. 现在来分析题目.题中的To……,是目的状语.而we 是主语, gear.to是谓语,而the qualitiesand varieties of products 是宾语,而we make 是来修饰这个宾语的,the world-market demand是宾补.句子的大致意思就是说我们必须根据市场的需求制造商品,才能在市场上立足.
5 选B regrettable 可惜的 regretful后悔的，抱歉的