Fuel were present at the sites. As a fuel, wood has remained in use up until the present day, although it has been superseded for many purposes by other sources. Wood has an energy density of 10–20 MJ/kg. Recently biofuels have been developed for use in automotive transport (for example Bioethanol and Biodiesel), but there is widespread public debate about how carbon efficient these fuels are. Fossil fuels are hydrocarbons, primarily coal and petroleum (liquid petroleum or natural gas), formed from the fossilized remains of ancient plants and animals by exposure to high heat and pressure in the absence of oxygen
Interlanguage fossilization on interlanguage fossilization are fascinating in that they contradict our basic understanding of the human capacity to learn. How is it that some learners can overcome interlanguage fossilization, even if they only constitute, according to Selinker, "a mere 5%", while the majority of second language learners cannot, "no matter what the age or amount of explanation or instruction"? Or is it perhaps not that they cannot overcome fossilization, but that they will not? Does complacency set in after second language learners begin to communicate, as far as they are concerned, effectively enough, in the target language, and as a result