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Fuel-efficient vehicle competition. Students from Mater Dei High School, Indiana, USA, preparing to compete in a fuel-efficient vehicle they have built during the 2015 Shell Eco-Marathon - an annual competition sponsored by Shell, in which participants build special vehicles to achieve the highest possible fuel efficiency. Photographed in Detroit, Michigan, USA, in 2015. (KEYSTONE/SCIENCE PHOTO LIBRARY/JIM WEST/SCIENCE PHOTO LIBRARY)