For the past 12 days, traders have paid more for options to buy the single currency versus the dollar than to sell it, the longest run since 2009, one-month volatility gauges show. The bulls anticipate flows back into the euro as Chinese and U.S. stock markets wrap up their worst month since at least 2010. The selloff may force investors who borrowed the European currency at historically low interest rates and sold it for foreign stocks to reverse their trades.

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