Stocks gave back the bulk of gains in the last half hour of trading in what had been a choppy start to the week. A surge in crude prices gave rise to the energy sector, while the health care and telecom sector were in retreat. Whole Foods  (WFM) jumped this afternoon after activist investor Jana Partners disclosed a stake of just over 8%. Jana is pushing for the supermarket chain to explore its options, including a possible sale. Swift Transportation  (SWFT) surged after agreeing to a stock-swap merger with Knight Transportation (KNX) . Both boards unanimously approved the roughly $5 billion. Amazon  (AMZN) clocked gains of more than 1% after Needham analysts raised their rating on the stock to BUY from HOLD. The firm said Amazon's market share in the U.S. could increase 16% over the next five years. 

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