On April 20, 2010, an explosion on the Deepwater Horizon Macondo oil well drilling platform released 4 million barrels (636,000 cubic meters) of oil into the Gulf of Mexico. It took scientists and engineers nearly three months and several different attempts to cap the well. The Macondo oil well was located 1524 m (5000 feet) below the surface of the water, about 80 km (50 miles) off the coast of Louisiana. The spill affected 2113 km (1300 miles) of shoreline along the coasts of five states. Most crude oil contains 84–87% carbon by weight. Scientists have not yet fully assessed the environmental impact of the accident. Reducing our dependence on petroleum is a difficult task, so policy makers are organizing it into smaller, simpler problems that can be solved more easily. Choose two of the solutions below that may help reduce our dependence on fossil fuels.
a
Researching alternative fuels to petroleum, such as ethanol.
b
Increasing taxes or fees to help pay for cleanup costs.
c
Requiring rooftop gardens to be built on all new buildings to reduce air pollution.
d
Developing transportation methods that do not require the use of petroleum products.

Mantle convection  is the slow  creepingmotion of earth's solid silicate  mantle  caused by  convection  currents carrying  heat from the interior  to the planet's surface.[1][2]  the earth's surface  lithosphere, which rides atop the  asthenosphere  (the two components of the  upper mantle), is divided into a number of  plates  that are continuously being created and consumed at their opposite  plate boundaries. accretion occurs as mantle is added to the growing edges of a plate, associated with  seafloor spreading. this hot added material cools down by  conduction  and  convection  of heat. at the consumption edges of the plate, the material has thermally contracted to become dense, and it sinks under its own weight in the process of  subduction  usually at an  ocean trench.[3]this subducted material sinks through the earth's interior. some subducted material appears to reach the lower mantle,[4]  while in other regions, this material is impeded from sinking further, possibly due to a phase transition from  spinel  to  silicate perovskiteand  magnesiowustite, an  endothermic reaction.[5]the subducted oceanic crust triggers  volcanism, although the basic mechanisms are varied. volcanism may occur due to processes that add buoyancy to partially melted mantle causing an upward flow due to a decrease in density of the partial melt.secondary forms of convection that may result in surface volcanism are postulated to occur as a consequence of intraplate extension[6]  and  mantle plumes.[7]it is because the mantle can convect that the  tectonic plates  are able to move around the earth's surface.[8]mantle convection seems to have been much more active during the  hadean  period, resulting in gravitational sorting of heavier molten  iron, and  nickel  elements and  sulphides  in the core, and lighter  silicate minerals  in the  mantle.