The correct answer is option 'A', which states that the Amur River separates China and Russia.

The Amur River, also known as the Heilongjiang in China, is one of the longest rivers in Asia, stretching over 4,444 kilometers. It originates in the hills of northeastern China, near the border with Mongolia, and flows eastward, forming the natural boundary between China and Russia.

Here is a detailed explanation of why the Amur River separates China and Russia:

1. Location:
- The Amur River marks a significant portion of the border between China and Russia. It runs from the confluence of the Shilka and Argun rivers, near the Russian city of Blagoveshchensk, and continues southeastward into China until it eventually reaches the Sea of Japan.

2. Border between China and Russia:
- The Amur River serves as a natural geographic boundary between China and Russia. It separates the two countries along its course, with China located to the south and Russia to the north.

3. Historical Significance:
- The Amur River has played a crucial role in the historical interactions between China and Russia. It has been a vital trade route, facilitating the movement of goods and people between the two countries for centuries.
- In the 19th century, the river became a focal point of territorial disputes between the Russian Empire and the Qing Dynasty of China. The Treaty of Aigun in 1858 and the Treaty of Peking in 1860 established the Amur River as the official border between the two powers.

4. Economic Importance:
- The Amur River basin is rich in natural resources, including timber, minerals, and fish. It supports a diverse array of flora and fauna, making it ecologically significant.
- The river also serves as a major transportation route for both China and Russia. It allows for the movement of goods, particularly timber and minerals, between the inland regions and coastal areas.

In conclusion, the Amur River serves as the natural boundary between China and Russia. It has historical, cultural, and economic significance for both countries and has played a crucial role in shaping their relationship over the years.