二 |     (ECNS) -- The central parity rate of the Chinese currency yuan strengthened 46 pips to 6.7808 against the U.S. dollar Thursday, according to the China Foreign Exchange Trade System.    In China's spot foreign exchange market, the yuan is allowed to rise or fall by 2 percent from the central parity rate each trading day.    The central parity rate of the yuan against the U.S. dollar is based on a weighted average of prices offered by market makers before the opening of the interbank market each business day.                        。