Wall Street opens lower as oil prices drop

Oil prices fell nearly 3 percent as China ramped up exports of refined products, U.S. producers added rigs for an eighth consecutive week, and prospects emerged for increased exports from Iraq and Nigeria. With the earnings season coming to an end, investor focus will shift to Federal Reserve Chair Janet Yellen's speech on Friday at the annual central bankers' meeting in Jackson Hole, Wyoming. The case for an imminent rate hike was strengthened by Fed Vice Chairman Stanley Fischer's comments tha