 Complete the following phrase:  In the early 20th century, a series of Sino-Japanese conflicts had raised Chinese peoples concern for national unity, and the concept of a unified, undivided Chinese nation became more popular among Chinese scholars. On Jan 1, 1939, Gu Jiegang published his article The term China proper should be abolished immediately, which argued that the widely accepted area covered by China proper is not the actual territory of any of the Chinese dynasties. Gu further theorized that , the Chinese and Japanese term equal to China proper at the time, actually originated from Japan and was translated into China proper, hence the concept of China proper was developed by Japanese people, and it had become a tool to divide Chinese people, making way for the Japanese invasion of Mongolia, 