阅读下面的文言文，完成后面题目。

    李清臣字邦直。七岁知读书，日数千言。客有从京师来者，与其兄谈佛寺火，清臣从傍应曰：“此所谓灾也，或者其蠹民已甚，天固儆之邪？”兄惊曰：“是必大吾门。”

    治平二年，试秘阁，欧阳修曰：“不置李清臣于第一，则谬矣。”时大雨霖，灾异数见，论者归咎濮^①议。及廷对，或谓曰：“宜以《五行传》‘简宗庙，水不润下’为证，必擢上第。”清臣曰：“吾不之信民间岂无疾痛可上者乎善止天地之异者不止其异止民之疾痛而已。”策入等，名声籍甚。英宗曰：“如李清臣者，公议皆谓可用，顾以亲抑之可乎？”既而诏举馆阁，欧阳修荐之，得集贤校理。

    绍圣元年，廷试进士，清臣发策曰：“今复词赋之选而士不知劝，罢常平^②之官而农不加富，赐土以柔远也而羌夷之患未弭，弛利以便民也而商贾之路不通。夫可则因，否则革，惟当之为贵，圣人亦何有必焉。”鄜延亡金明寨，主将张舆战没，章惇怒，议尽戮全军四千人。清臣曰：“今悉诛吏士，异时亡将必举军降虏矣。”于是但诛牙兵十六辈。仆射韩忠彦与之有连，惟其言是听，出范纯礼、张舜民，不使吕希纯、刘安世入朝，皆其谋也。寻为曾布所陷，出知大名府而卒，年七十一。赠金紫光禄大夫。

    清臣蚤以词藻受知神宗，建大理寺，筑都城，皆命作记，简重宏放，文体各成一家。为人宽洪，不忮害。尝为舒亶所劾，及在尚书，亶以赃抵罪，独申救之，曰：“亶信亡状，然谓之赃则不可。”起身穷约，以俭自持，至富贵不改。然志在利禄，一意欲取宰相，故操持悖谬，竟不如愿以死。

（选自《宋史·李清臣传》，有删改）

【注】①濮：指濮王。②常平：常平仓。常平之官，简称“仓臣”，掌常平仓、免役、市易、坊场、河渡、水利等事。

     What's going to happen in the future? Will robots control our planet? Will computers become smarter than us? Not likely. But here are some things that scientists say are most likely to happen in 10 to 30 years from now, according to the BBC.

     Digital money

     We used to pay with cash(现金) for everything we bought. Now when we use a credit card (信用卡)  to shop online, money is spent without us seeing it. That means we are already using digital money. Using a card is much easier than searching our pockets for change. It is also safer than carrying a lot of cash.

     When ATM cards were first introduced, they were not accepted everywhere. But now it’s hard to live without them. It's reposed that people in Sweden completely stopped using cash last year, and the US might be next.

     Bloke (仿生的) eye

     It's no longer something only in a science fiction movie. People who are blind may have a chance to get their sight back-by wearing bionic eyes.

     A blind eye can no longer sense light, but a bionic eye can use a camera to “see” the environment and send data(数据) to the mind. Now the bionic eye only allows patients to see lights and unclear shapes. A high resolution (分辨率) image cold be just a few years away.

     Self-driving cars

     Unlike a human driver, a self-driving car won't get distracted(分神的)by a phone radio or something outside the window. Sensors (探测器) and cameras on the car would allow it to keep strictly to the rules of the road and keep a safe distance from other cars. This would greatly reduce the number of road accidents. You could even take a nap while the car drives itself. In the future, driverless cars would be widely accepted.

将凸透镜正对太阳光，其下方的纸上呈现一个并非最小的光斑，这时光斑到凸透镜的距离为L。若凸透镜远离纸的过程中光斑一直变大，该凸透镜的焦距(    )