A wildly popular oil ETF has been cut off by its broker from buying futures, limiting its ability to meet its investment objective (USO)

· *USO — an oil exchange-traded fund that's wildly popular with traders — has been cut off from buying oil futures contracts after a surge in volatility saw oil prices turn negative last month.*
· *RBC Capital Markets has informed United States Commodity Funds, the operator of the USO ETF, that they may no longer hold...