Bitcoin is offering a decentralisation system for which there is no reason for anyone could have the control over it. Though its a fact all the other big mining institutions are in China but, it's also a fact that there are thousands of thousands CPU are occupied by which BTC also mining from all over the world and helping blockchain technology to work smoothly.
I a believe that China can't control bitcoin. Even though they have a largest farm all over the world. China can only be one of the biggest investors who adopted the system of bitcoin. meaning they're just one of the member in the community investors. I don't think they control the bitcoin because there has been many other exchanges rather in china which have a great volume of bitcoin too but china is leading in this field and has the power to manipulate the price of bitcoin.