Which of the following describes a positive feedback loop? When blood sugar concentration is too low, the endocrine system secretes hormones that increase blood sugar concentration to a normal level.

When the body senses that the heart rate is very high, it secretes hormones that decrease the heart rate.

When blood sugar concentration is too high, the endocrine system secretes hormones that decrease blood sugar concentration to a normal level.

When someone is exercising intensely, the body secretes hormones that increase oxygen supply for cellular respiration.

a fossil fuel is a fuel formed by natural processes, such as anaerobic decomposition of buried dead organisms, containing energy originating in ancient photosynthesis.[1] the age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years.[2] fossil fuels contain high percentages of carbon and include petroleum, coal, and natural gas.[3] other commonly used derivatives include kerosene and propane. fossil fuels range from volatile materials with low carbon to hydrogen ratios like methane, to liquids like petroleum, to nonvolatile materials composed of almost pure carbon, like anthracite coal. methane can be found in hydrocarbon fields either alone, associated with oil, or in the form of methane clathrates.

the theory that fossil fuels formed from the fossilized remains of dead plants by exposure to heat and pressure in the earth's crust over millions of years was first introduced by andreas libavius "in his 1597 alchemia [alchymia]" and later by mikhail lomonosov "as early as 1757 and certainly by 1763".[4] the first use of the term "fossil fuel" was by the german chemist caspar neumann, in english translation in 1759.[5]

in 2017 the world's primary energy sources consisted of petroleum (34%), coal (28%), natural gas (23%), amounting to an 85% share for fossil fuels in primary energy consumption in the world.[6] non-fossil sources in 2006 included nuclear (8.5%), hydroelectric (6.3%), and others (geothermal, solar, tidal, wind, wood, waste) amounting to 0.9%.[7] world energy consumption was growing at about 2.3% per year. in 2015 about 18% of worldwide consumption was from renewable sources.[8]

picture a world where computers are building made-to-order personalized robotic arms and synthetic organs, drones are circulating above mars! due to the rapid growth of technology, the future has never been more unpredictable, and it presents a huge challenge in education: we are teaching students how to work in a future that we have never been unsure of. it is widely predicted that in eight years’ time, there will be more jobs in the fields of science, mathematics, technology and engineering than in any other discipline. and, as the technology continues to develop, it will soon become clear that addressing social problems will involve more innovative thinking, and analytical and technical knowledge than ever before. undeniably, there are no subjects that encourage innovative and creative thinking, deep thought, problem solving and independence more than the stem subjects. while it is widely commented on that there is a need to reimagine the ways stem subjects are taught in schools by embracing a more interactive and innovative approach to appeal to more students, it should come as no surprise that some of the most reputable educational organizations in the world are already implementing changes to embrace stem. harvard, for example, announced in 2014 that they were increasing their computer science faculty by over 50% and has since introduced computer science as a core subject for all freshmen. the urgency of supporting stem subjects at a young age is becoming more relevant than ever, as adopting a keen interest in the curricula needs to happen early. however, beyond the age issue is one of gender.

let me know if you need more