The main reason early Chinese civilization and dynasties thrived for thousands of years with relatively little outside influence is primarily due to c. mountains and deserts provided natural barriers. These geographical features, such as the Himalayas to the southwest, the Taklamakan Desert to the west, and the Gobi Desert to the north, acted as significant obstacles to invasion and interaction with other cultures.
While the Great Wall (option a) was built later to enhance these barriers and protect against invasions, it was the natural geography that initially provided protection. Option b, claiming that the Chinese did not need to trade, is not entirely accurate, as trade routes like the Silk Road did facilitate some exchange with outside cultures. Option d, suggesting that China is protected by water on all sides, is also misleading, as China has land borders with several countries.
So, the correct answer is c. mountains and deserts provided natural barriers.