By the 1800s, China had become 4. because of internal problems.

By the mid-nineteenth century, China's population reached 450 million or more, more than three times the level in 1500. The inevitable results were land shortages, famine, and an increasingly impoverished rural population. Heavy taxes, inflation, and greedy local officials further worsened the farmer's situation.

answer; i believe the correct answer is ; large pieces of pangaea drifted in different directions over millions of years;