18世纪英王派使者来华,提出在中国建立商馆、自由贸易的要求。乾隆皇帝回复:“天朝物产丰盈,无所不有,原不藉外夷货物以通有无。”这反映出清朝实行的政策是（    ）

    Have you ever been to the world's smallest bookstore?

    The World's Smallest Bookstore, whose official name is just these three words, sits quietly about 100 miles northeast of Toronto.

    The bookstore is about 10 feet by 10 feet，so it is easy to imagine how tiny it really is. The bookstore is open 24 hours a day. Inside the bookstore are various books, especially literary books and classic authors' works. So if you are looking for something less popular, you may get a bit disappointed there.

Another special feature of this bookstore is that each book only costs three dollars. All the expenses are paid on the honor system, which means buyers should make a note of what they've bought and leave their money by themselves. So the tools of the trade in this bookstore are quite simple: pens, papers, light bulbs and a label-maker.

    In order to catch passers-by's attention, the billboards (广告牌) of the bookstore are several times bigger than the store itself. With these large eye-catchers, many people are willing to stop by and have a visit.

如图所示，一质量m=4.0kg的小球在轻质弹簧和细线的作用下处于静止状态，细线AO与竖直方向的夹角θ=37°，弹簧BO水平并处于压缩状态，小球与弹簧接触但不粘连，已知弹簧的劲度系数k=100N/m，取sin37°=0.6，cos37°=0.8，求：