U.S. stocks opened mostly lower on Thursday, as ongoing uncertainty surrounding trade policy weighed on investor sentiment. The S&P 500 was flat at the open at 2,700. The Dow Jones Industrial Average fell 38 points, or 0.2%, to 24,084. The Nasdaq Composite Index declined 8 points, or 0.1%, to 7,436 at the open. Among the biggest losers on the S&P 500, shares of Walgreens Boots Alliance Inc and CVS Health Corp dropped nearly 10% after Amazon.com announced that it has agreed to acquire PillPack, an online pharmacy that pre-sorts medications into different doses and handles both refills and renewals.