在用等效替代法测量未知电阻的实验中，其中R_x是待测电阻，R是滑动变阻器，R₀是电阻箱（电阻箱的最大电阻值大于R_x。）

请根据实验电路图把下列主要实验步骤中的空白填全。

⑴按电路图连好电路，并将电阻箱R₀的阻值调至最大；

⑵闭合开关S₁前，滑片P置于____端（选填“a”或“b”）；

⑶闭合开关S_1；S₃，调节滑片P，使电流表指针指在适当的位置，记下此时电流表的示数I；

⑷先断开开关S₃，再闭合开关S₂，保持____的电阻不变，调节____，使电流表的示数仍为I，此时电阻箱面板如图所示，则 R_x=____。

. By the time you arrive in London, we        in Europe for two weeks.

下列各数：0.3333…，0，4，﹣1.5， $\frac{\pi }{2}$ ， $\frac{5}{3}$ ，﹣0.525225222中，无理数的个数是（   ）

阅读理解
    Bargaining is the rule here in Beijing. At least, it is so in most markets and back—street clothes shops.  Bargaining is an art and if you are unfamiliar(不熟悉) with it, we'd like to offer you some advice. The tips(窍门) here are often used in Beijing but may help you at any place in the world where bargaining is practiced.
    DO NOT say how much you want to pay for an item(物品) unless it's near the end of the bargaining. Always try and drop the seller's offering price as much as possible before opening your mouth with a price.
    DO throw out really low prices like 10 RMB with a big smile.
    DO keep smiling from the beginning to the end. The seller usually continues bargaining with a happy smiling face. Getting angry hardly gets you the price you want.
    KEEP in mind the price offered by the seller at the beginning is usually at least 40% over the generally reasonable price. It can be up to 500% over.
    DO have an idea of what the item costs. You can ask your friends, people in the hotel or others you know, or look at the list below. This is very general and is based(根据) on a market like Xiu Shui. You may not be able to get the lowest prices at Xiu Shui, especially on a weekend when there are lots of tourists around.

窑洞这种民居分布在（   ）