请先阅读下面的短文，掌握其大意．然后在每小题所给的A，B，C，D四个选项中，选出可以填入空白处的最佳选项。

    Today some people call Amsterdam the "City of Bicycles" because it is a city which is flat and convenient（方便）for bicycles.

    In the 1960s, a group of cycling fans1an idea . They believed that it would be better for everybody if only bicycles were allowed in the city centre. They were2that this would help to save energy, reduce pollution and provide free public transport. The group painted hundreds of bicycles3and placed them in public places around Amsterdam for people to use.4was allowed to take them and use them for short journeys, whether he was a local or a foreigner, Wherever someone finished a journey, they would5the bike there for someone else to use. The problem was that it didn't work-thieves took all the bicycles6 weeks!

    7,more than thirty years later, the "white bike" is back in town-this time with a computer chip(芯片)to8 its every move! To take a bicycle, you have to put a special card inside. The new "White bike" is not white any more but is an unusual9with bright colours. The bikes are parked at special parking places and people who want to use them have to take them to another place that has enough room.

    There is already less traffic in central Amsterdam, 10 both locals and tourists have been using the white bikes. Thanks to the good ideas of lots of people, like the cycling fans in the 1960s, many people around the world have been enjoying city centre streets without cars for many years.

下列化学用语表示正确的是（   ）

当k= ____时，方程组 中x与y互为相反数．

7世纪，日本与阿拉伯帝国面对社会危机，选择了独特的发展道路。阅读材料，回答问题。

材料一：大化5年（公元649年），孝德天皇召集群臣盟誓“天覆地载，帝道唯一”，并建立“置八省（指民部省、兵部省、刑部省）百官”的中央机构和地方行政机构（国、郡、里）。

——摘自华师大版九年级世界历史教科书

材料二：现在，他给它带来了一个宗教和一部天启的经典，带来了一个具有良好的组织和武装、统治了整个半岛的社会和国家。一个世纪内，他的信徒们从胜利走向胜利，建立了一个横跨欧亚大陆的强大帝国，并传播了他的教义。如今，这一宗教在世界上拥有5亿信徒。如果对事态发展的影响为标准，那么，默罕默德无疑是历史上的一位巨人。

——斯塔夫里阿诺斯《全球通史》