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China has a very long history. It has many great traditional skills that have a widely influence(影响). Here we mainly talk about kites.

  The earliest mention of a kite in ancient China dates back to the 5th century BC. It was made of wood and used as a rescue signal(求救信号).

  At the time between 770 and 221 BC, Chinese kites were actually called “muyuan”. They were first invented in Shandong Province for military purposes(军事目的). The kites were used to send important messages during the Chu-Han War(203-202 BC). During the Tang Dynasty(618-907 AD), kites were made of lighter materials, such as bamboo, silk and paper. At that time, people began to fly kites for other purposes. It was believed that letting go of the kite could remove bad luck and illness.

  Today, kite-flying is an interesting and healthy activity in China. There are large kite-flying festivals held throughout the country, and kite makers can show off their skills during the festivals.

图中B点的海拔是____米，A山顶与B山顶的相对高度是____米。

1949年签署的《北大西洋公约》第二条规定：“缔约国应消除其国际经济政策中之冲突，并鼓励任何缔约国或所有缔约国之间的经济合作。”该条规定对于美国来说（   ）