China has no way to go, but to buy US gov bonds

China's rmb is pegged with US currency, so when any people is present on China's central bank's counter, asking to exchange Dollar for RMB, China has to take that dollar and produce the amount of RMB according to exchange rate, when people wants to exchange RMB for dollar, China has to produce US dollar. How China is going to produce dollar? China do it either from under its big mattress which produce no interest, or China sells US bond and get dollar. Remember one thing that China is not the majority owner for its dollar reserve, foreign direct investment and foreign company/Chinese company's international trade surplus forms those reserve, China's central bank need to be ready to be able to provide enough liquidity for those real owners to freely exchange dollar and RMB on a daily basis, it needs to have enough liquidity to prevent a bank run on its dollar reserve, where else can China's central bank put us dollar other than US gov bonds which provides the best liquidity plus some interest.