(Reuters) A fund trading strategy that tracks hundreds of billions of dollars in assets and often gets blamed for exacerbating market selloffs is facing a challenge from the policy response to the pandemic. But fund managers said they are adapting, and new money is flowing in. Most closely associated with investment funds such as Bridgewater Associates and AQR, the strategy, called risk parity, typically spreads risk over stocks, bonds and other financial assets, as against traditional 60-40 stock and bond portfolios where equities carry more risk.