这是智能手机发展中的巨大里程碑，如今大多数美国成年人都拥有智能手机，这种情况在全球范围内也变得越来越普遍。

It was a huge milestone in the development of smartphones, which are now owned by a majority of American adults and are increasingly common across the globe.

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京张高速铁路于2019年12月开通。这条铁路是中国铁路发展的一个里程碑，因为它采用了铁路建设的最新技术。

The opening of the Beijing-Zhangjiakou high speed railway took place in December, 2019. The railway is a milestone in China's railway development because it has used the latest technologies in railway building.

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近年来，我国的公路建设飞速发展，高等级公路通车里程快速增加。

In recent years, China's rapid development of highway construction, high grade highway mileage increase rapidly.

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