In this sentence: The birth rate in the USA was higher than the birth rate in China. If I wanna replace "the birth rate" before "in China", I would use "that" or "one"? Actually, I have already referred to some sample essays, the authors always use "that", but I do not know why they do not use "one"? Thank you so much!
To keep your sentence structured the same way, the best wording would be: The birth rate in the USA was higher than that in China.
Using the word one instead of that could be confusing or misleading. A country can only have a single birth rate. Using the word one makes is sound like China could have multiple birth rates at the same time.