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    The bike-sharing companies that are powering a transportation revolution in China's urban centers are not like those in the rest of the world. Their bikes have no docking ports or assigned zones for parking. Instead, riders open their smartphone apps, hop on, and start riding. So easy!

    In 2015, Ofo, China's bike-share pioneer, launched its first fleet of bicycles in Beijing. The bikes' bright yellow frames were soon met by the orange-rimmed(镶边), basket-equipped models launched by Mobike, Ofo's main competitor today. In little over a year, the two companies have had a huge impact on revitalizing (重振) cycling in China, particularly among young people.

    Ofo placed 1 million bicycles across 35 cities and registered 15 million users in its first years, while Mobike has now gained more than 10 million unique users and bicycles in 21 cities. Inspired by their success, at least 10 other companies have jumped into the market, launching their own app-powered, brightly colored bikes in quick succession. But Mobike and Ofo are far ahead of them.

    The two industry leaders each take a different marketing method to a similar product. The Mobike app uses GPS to display bikes' locations to users. They can use the map to reserve a bike and walk over to it. A scan of the bike's QR code (二维码) unlocks the bike, and when users manually(手动地) lock the bike at the end of their trip, the app records the trip's end and the bike will pop back up on the map as available. Equipped with this wireless technology, Mobikes cost between 1,000 and 3,000 yuan. Rides cost 1 yuan for an hour or a half hour, depending on whether a user chooses a “Lite” or regular bike, and the deposit (押金) required to the service is 299 yuan.

    Ofo, meanwhile, is devoted not just to producing its own bikes, which have slim yellow bodies and cost about 250 yuan, but also to connecting existing bikes to its network----- a philosophy that begins with the practice of 2000 bikes offered to a campus bike-share program. The 2000 bikes were the first generation of Ofo. Yet the majority of the company's bikes are currently ones that have been specially manufactured and fitted with their lock system. Unlike Mobike,Ofo bikes themselves are offline: their locations are tracked through users' cell phones. Users find a free bike, enter the license plate number into their app, and receive the bike unlock code. When they have reached their destination, they end the ride on the app and manually lock the bike. Rides cost 1 yuan for an hour and the deposit is 99 yuan.

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小李同学为测量某金属丝的电阻率,他截取了其中的一段,用米尺测出金属丝的长度L,用螺旋测微器测得其直径为D,用多用电表粗测其电阻约为R.

①该同学将米尺的0刻度线与金属丝的左端对齐,从图(甲)中读出金属丝的长度L=____cm.

②该同学用螺旋测微器测金属丝的直径,从图(乙)中读出金属丝的直径D=____mm.

③该同学选择多用电表“×10”档粗测金属丝的电阻,从图(丙)中读出金属丝的电阻R=____Ω.

④接着,该同学用伏安法尽可能精确地测出该金属丝的电阻,实验电路如图丁所示,然后根据电阻定律计算出该金属丝的电阻率.实验室提供的器材有:

A.直流电源E(电动势4V,内阻不计)

B.电流表A1(量程0~3mA,内阻约50Ω)

C.电流表A2(量程0~15mA,内阻约30Ω)

D.电压表V1(量程0~3V,内阻10kΩ)

E.电压表V2(量程0~15V,内阻25kΩ)

F.滑动变阻器R1(阻值范围0~15Ω,允许通过的最大电流2.0A)

G.滑动变阻器R2(阻值范围0~2kΩ,允许通过的最大电流0.5A)

H.待测电阻丝Rx,开关、导线若干

要求较准确地测出其阻值,电流表应选____,电压表应选____,滑动变阻器应选____.(用器材前的字母表示即可)

⑤用图所示的电路进行实验测得Rx,实验时,开关S2应向____闭合(选填“1”或“2”).

⑥请根据选定的电路图,在如图戊的实物上画出连线____(部分线已画出).

⑦设金属丝的长度为L,测得直径的平均值为D,测得的电压值是U,电流值是I,则金属丝电阻率的表达式为ρ=____(用给定字母表示)