Petroleum, or crude oil, is one of the world's __＿＿（56）＿＿__ natural resources. Plastics, synthetic fibers, and __＿＿（57）＿＿__ chemicals are produced from petroleum. It is also used to make lubricants and waxes. However, its most important use is as a fuel for heating, for __＿＿（58）＿＿__ electricity, and especially for powering vehicles.