CHICAGO, IL - FEBRUARY 06:  Traders signal offers in the S&P options pit at the Cboe Global Markets, Inc. exchange (previously referred to as CBOE Holdings, Inc.) on February 6, 2018 in Chicago, Illinois. Yesterday the S&P 500 and Dow Industrials indices closed down more than 4.0 percent, the biggest single-day percentage drops since August 2011.  (Photo by Scott Olson/Getty Images)