Synthetic fuel the fuel, and the fuel is less dense than conventional jet fuel allowing aircraft to fly further on the same load of fuel. The "cleanness" of these FT synthetic fuels is further demonstrated by the fact that they are sufficiently non-toxic and environmentally benign as to be considered biodegradable. This owes primarily to the near-absence of sulfur and extremely low level of aromatics present in the fuel. One concern commonly raised about the development of synthetic fuels plants is sustainability. Fundamentally, transitioning from oil to coal or natural gas for transportation fuels production is a transition from one inherently depletable
Interlanguage fossilization 1) why fossilization occurs, 2) the precipitating conditions, 3) what kind of linguistic material is likely to be fossilized, and 4) what type of learners are more prone to fossilize. However, there has been almost no investigation by SLA theorists on the possibilities of preventing or overcoming fossilization, and little explanation related to those adult second language learners who overcome one or more "areas of stability" in interlanguage—those learners whose interlanguage does not fossilize, and who reach a high level of proficiency in the second language. One factor of obvious relevance is motivation, and studies have been conducted regarding motivation