1. (2020·西城模拟) 阅读短文，根据短文内容填空。在未给提示词的空白处仅填写1个适当的单词，在给出提示词的空白处用括号内所给词的正确形式填空。

    Nearly two decades have passed since China sent the first Beidou satellite into space in 2000. During that time, more Beidou satellites were sent into orbit (轨道),  (form) the Beidou Navigation Satellite System (BDS). In late 2012, it began providing positioning, navigation, timing and messaging  (service) to people in China and other parts of the Asia-Pacific region. At the end of 2018, BDS started to serve users worldwide. Now with two more Beidou satellites  (launch) on Dec 16, 2019, BDS has 53 satellites in orbit.