3 sentences

1. 秦岭是中国南北方的地理分界线。

   Qín lǐng shì Zhōng​guó nán​běi​fāng de dì​lǐ fēn​jiè​xiàn.

   The Qinling Mountains are the geographical divide between northern and southern China.

   Source: Dong Chinese Dictionary

2. 淮河是中国南北方的一条重要地理分界线。

   Huái hé shì Zhōng​guó nán​běi​fāng de yī tiáo zhòng​yào dì​lǐ fēn​jiè​xiàn.

   The Huai River is an important geographical divide between northern and southern China.

   Source: Dong Chinese Dictionary

3. 这条漫长的山脉成为了两个地区的天然分界线。

   zhè tiáo màn​cháng de shān​mài chéng​wéi le liǎng gè dì​qū de tiān​rán fēn​jiè​xiàn.

   This long mountain range has become a natural boundary between the two regions.

   Source: Dong Chinese Dictionary