The USA has been the strongest economy in the world since the Second World War. It possessed almost two thirds of the global gold reserves which gave it immense financial strength and consequently legitimized the US Dollar as a Global Reserve Currency. The same confidence is being shown by the yuan as Pakistan has reportedly paid yuan to Russia in return for crude oil. These are all prerequisites for a currency to attain global reserve status and China certainly possesses a leverage in that regard. It has lent military and economic support to Pakistan and has been its most reliable diplomatic partner as well, especially in the UN.
