a₁= $\frac{1}{2}$ ，a₂= $\frac{1}{1-a_1}$ ，a₃= $\frac{1}{1-a_2}$ ，…，a_n= $\frac{1}{1-a_{n-1}}$ （n≥2且n为正整数），则a₂₀₁₇的值为____（结果用数字表示）

阅读下列短文, 从每题所给的四个选项（A、B、C和D）中选出最佳选项。

"Dreams (梦) may be more important than sleep. We all need to dream," some scientists say. Dreams take up about one quarter of our sleeping time. People have several dreams each night. Dreams are like short films. They are usually in color. Some dreams are like old films. They come to us over and over again. That may be because the dreamer is worrying about something. Dreaming may be a way of trying to find an answer.

    Some people get new ideas about their work from dreams. They may have been thinking about their work all day. These thoughts can carry over into dreams.

    Sometimes we wake up with a good feeling from a dream. But often we can't remember the dream. Dreams can disappear quickly from memory.

    Too much dreaming can be harmful. The more we sleep, the longer we dream. The mind is hard at work when we dream. That is why we may have a long sleep and still wake up tired.

材料一：明代内阁自始至终只是皇帝的私臣，只是政务的补助机关，一切公文都以皇帝名义发布，但内阁制度在明代仍有一定程度的作用。太祖废相后，君主成为实际的行政首长，一切大政均须亲力亲为。……虽然在辅政方面，确能发挥积极的作用，但明代内阁大学士地位不足当宰相之任，且其本职亦只是侍从备顾问。后来内阁获取票拟之权，亦只是在幕后协助皇帝处理国政，属皇帝秘书而不是名正言顺的宰相，……就权责而言，内阁始终不是法定的最高行政机构，首辅亦非法定的最高行政首长，六部也不是它的法定直接下属，而由君主独掌命令大权。内阁所持有的权力，出自皇帝授予。因此，内阁有否作为，全视皇帝的作风而定，故历朝内阁的权力都有所不同。

——摘自《试述明代内阁制度之由来、演变、作用、流弊及影响》

材料二：（西方）内阁最早出现于英国，……16世纪～18世纪初的内阁还只是处于萌芽状态，最初只是国王与大臣共商国事的会议。一直到英国的乔治一世担任国王期间，由于不会说英语，开始很少参加内阁会议，反而由大臣主持，这样就首开英国内阁制的先声。1721年，英国议会下院多数党领袖沃波尔出任内阁首相，内阁正式形成，后来不断发展完善。英国的内阁主要由首相提名成员名单，国王批准成立，内阁名义上对国王负责，但实际上要对议会负责，……内阁由首相主持，是英国政府的核心，是行政决策机构。

——摘自《浅议中英内阁制度》