U.S. oil prices have plunged into negative territory for the first time in history. They ended at nearly minus 38 U.S. dollars a barrel on Monday, before rebounding later. U.S. President Donald Trump said he is considering the possibility of stopping incoming Saudi Arabian crude oil shipments. Professor Liu Baocheng from the University of International Business and Economics in Beijing says the stabilization of U.S. oil prices should be achieved by output coordination.